The GBT IF/LO System

October 15, 1999

*** New *** (19April2005)

Block diagrams of the IF/LO system in pdf format.

1.0 Introduction

The GBT IF/LO system consists of the electronic subsystems and transmission lines used to connect receiver front-end outputs with data acquisition backends. This memo will concentrate on the common IF chain, which accepts an IF signal in the 1-8 GHz frequency range from each front-end, and produces output IF signals at frequencies appropriate for the various backends. Included are frequency synthesizers and other components of the local oscillator (LO) system, the operation of which is intertwined with the operation of the mixers incorporated in the receiver front-ends, and in frequency converters in the common IF chain.

Section 2 of this memo describes the Receiver Room LO Rack equipment and the LO Reference Distribution system. Section 3 describes the Receiver Room IF Distribution, Section 4 the Equipment Room IF Distribution equipment, and Section 5 the frequency conversion formulas and frequency constraints for each of the GBT front-ends. Section 6 gives IF interface information for the GBT backends, and Section 7 some examples of equipment setups.

In each of the installed front-ends (on the Gregorian turret or on the prime focus boom) a frequency conversion is performed to produce the first IF signal. Table 1-1 lists the first IF center frequency and bandwidths for the GBT receivers. The mixer for the first conversion is located inside the prime focus front-end box or on a plate attached to the Gregorian front-ends' dewar, and the first mixer local oscillator signal is provided by the system LO1 synthesizers (via the LO Router described in Section 2).

Coaxial cables connect the first IF signals to the IF Router, located in the IF Rack on the turret center post. The IF Router allows the user to select several of the IF signals for connection to Optical Fiber Driver modules which transmit the signals over optical fibers to the Equipment Room located off the antenna.

At the Equipment Room, the optical fibers connect to Optical Receiver modules in which the IF signals are demodulated, amplified, and split for distribution. The Optical Receiver module outputs connect to Converter Modules. The Converter Modules, with associated local oscillators, are tunable over the 1-8 GHz input frequency range, and provide switch selectable outputs for various supported backends.

         RX     GHZ         1st LO     1st IF Band                      
     No     LOW   HIGH  Range      Cen/BW (GHz)   Polarization  

     P-1   0.68   0.92  1.76-2.00  1.08/0.02,...  Dual Lin/Circ
     P-2   0.51   0.69  1.59-1.77  1.08/0.02,...  Dual Lin/Circ
     P-3   0.39   0.52  1.46-1.60  1.08/0.02,...  Dual Lin/Circ
     P-4   0.29   0.40  1.37-1.48  1.08/0.02,...  Dual Lin/Circ

       1   1.15   1.73  4.15-4.50  3/0.6          Dual Lin/Circ
       2   1.73   2.60  7.73-8.60  6/1            Dual Lin/Circ
       3   2.60   3.95                            Dual Lin/Circ
       4   3.95   5.85  6.9-9.0    3.2            Dual Lin/Circ
       5   5.85   8.20                            Dual Lin/Circ

       6   8.00  10.00  11-13      3/2, 3/0.5     Dual Circular
       7  10.00  12.40                            Dual Circular
       8  12.00  15.40  9.0-12.5   3/3.5,3/0.5    Dual Circular
       9  15.40  18.00                            Dual Circular
      10  18.00  22.00  14.5-17.5  6/4            Dual Circular

      11  22.00  26.50  16-8.5     6/4.5          Dual Circular
      12  26.50  33.00                            Dual Circular
      13  33.00  40.00                            Dual Circular
      14  40.00  52.00  8.5-11(X4)                Dual Circular

2.0 LO System

Included within the LO system is the Receiver Room LO Rack, a LO Reference distribution system, and components located within the Equipment Room.

The Receiver Room LO Rack is shown in Figure 2-1, and the block diagram is shown in D35260K003. Components consist of two LO1 synthesizers, the LO Router, and the Test Tone Router. These generate LO signals for the receiver front-ends and allow the signals to be connected to the proper front-end ports. In addition, the Test Tone router allows the user to inject test signals into the front-ends for system checkout. Detailed descriptions of the operation of this equipment is provided in A35260D001, "GBT LO System Monitor/Control".

The LO Reference Distribution system, also shown in D35260K003, accepts a 100 MHz and 10 MHz signal from the hydrogen maser standard located in the Timing Center, multiplies the 100 MHz to produce 500 MHz, and transmits 500 and 10 MHz signals using single-mode optical fiber over separate links to the GBT Equipment Room and to the GBT Receiver Room. There 500, 10, and 5 MHz (generated by dividing the 10 MHz signal) are distributed. In addition, a 500 MHz signal is returned over separate fibers to the Timing Center where Round-Trip Phase modules are used to measure the round-trip delay for the 500 MHz signal. The measured delay is available via the GBT M/C system.

Components of the LO system located in the Equipment Room are described in the following sections in conjunction with the IF system.

3.0 Receiver Room IF Distribution

Receiver Room IF connections are illustrated in D35260K004. Components of the common IF system are incorporated in the Receiver Room IF Rack (RNE3 Unit 2) as shown in Figure 3-1. The rack is 48 inches high and mounts directly to the feed turret central shaft. Descriptions of the major IF subsystems in the Receiver Rack follow.

3.1 Receiver Room IF Router (RNE3 2A2)

The IF Router block diagram is included in D35260K004. The Router has sixty-four SMA IF input connectors located on a plate at the top of the turret IF rack. The IF inputs connect, in groups of eight, to eight SP8T switches (S1-S8). Transfer switches (S9-S12) at the outputs of pairs of the SP8T switches provide some redundancy should an IF link following the Router fail, and provide a convenient means to interchange channels for testing. The power dividers (PD1-PD8) provides a monitor port for each channel on a plate at the front of the Router. The Router IF outputs connect to the Optical Fiber Driver modules.

The microwave switches utilize solid-state PIN diodes for stability, speed, and reliability. Microwave components in the IF Router are specified to operate over the 0.1-8.0 GHz range. In current receiver plans, all front-ends produce first IF signals within the 1.0-8.0 GHz range. The extra bandwidth below 1 GHz is provided in the IF Router in case the need arises in the future to transmit IF signals in that range.

The Router IF input connectors provide a convenient location to reconfigure the system manually if required. For instance, an IF cable could be moved from input IF-2 to IF-33. Although this requires a trip to the Receiver Room, it could prove useful for unusual programs such as dual-frequency observing, the installation of new receivers, tracing unusual problems, or other special cases.

3.2 Optical Fiber Driver Module (RNE3 2OD1-2OD8

A block diagram of this module is shown in C35260K006. Eight Optical Fiber Driver modules are provided. These provide amplification and level-setting of the IF signals, contain the microwave optical laser transmitters, and include a square-law detector to monitor the IF power level and for broadband continuum observations. The IF input is via a SMA connector, and optical output is via a FC/APC connector, both on the module rear panel. On the front panel, LEDs indicate the step attenuator setting and a digital panel meter indicates the detected total power.

The optical laser transmitter has a rather high noise level, so the IF signal should be maintained above the recommended minimum input level (-35 dBm). The step attenuator (AT1) and the detector power monitor provide means to adjust the transmitter input power. The step attenuator (AT2) is a GaAs MMIC type for stability, speed, and reliability. Microwave components in the Optical Fiber Driver module are specified to operate over the 0.1-8.0 GHz range. In current receiver plans, all front-ends produce first IF signals within the 1.0-8.0 GHz range. The extra bandwidth below 1 GHz is provided in the Optical Fiber Driver modules in case the need arises in the future to transmit IF signals below 1 GHz.

The IF bandwidth received at the total power detector monitor is determined by bandpass filter(s) in the front-end. No additional filtering is provided in the IF Router or Optical Driver modules.

3.3 Noise Source Driver Module (RNE3 2NS1)

A single module that generates an amplified broadband noise spectrum is provided for IF system diagnosis/test purposes. The block/wiring diagram of this module is shown in C35260S003. A power splitter at the output allows up to four IF Router inputs to be driven at once. The user may select either a broadband (0.1-8 GHz) or narrowband (2.75-3.25 GHz) output spectrum. Both deliver the same integrated output level (approximately -25 dBm).

4.0 Equipment Room IF Distribution

The eight optical fibers driven by the Optical Fiber Driver modules go from the Receiver Room through the antenna elevation and azimuth cable wraps to the Equipment Room. Drawing C35260K009 shows a block diagram of the IF distribution within the Equipment Room. IF and LO equipment is distributed between three racks: Converter Rack A (RNG3 Unit2), Converter Rack B (RNG3 Unit 3), and the Analog Filter Rack (RNG3 Unit 4). The IF fibers connect to Optical Receiver modules (2OR1-2OR4, 3OR5-3OR8) located in the Converter Racks. These modules demodulate the optical signal, and amplify and split the IF1 signal for distribution to converter modules (2CM1-2CM8, 3CM9-3CM16) in the same racks. The converter modules upconvert IF1 to about 9.5 GHz with tunable local oscillator (2G1-2G4, 3G5-3G8) and then downconvert to a third IF with several output ports for the various backends. More details on each of the modules follow.

4.1 Optical Receiver Module (2OR1-2OR4, 3OR5-3OR8)

A block diagram of this module is shown in C35260K008. The broadband photodiode receiver (A1) demodulates the amplitude-modulated optical signal. The resulting electrical signal is amplified (A2) and split (E1) to the four outputs (J2-J5). Microwave components in the Optical Receiver module are specified to operate over the 0.1-8.0 GHz range. In current receiver plans, all front-ends produce first IF signals within the 1.0-8.0 GHz range. The extra bandwidth below 1 GHz is provided in the Optical Receiver modules in case the need arises in the future to transmit IF signals below 1 GHz. There are eight modules provided, producing thirty-two IF output signals.

4.2 1-8 GHz Converter Module (2CM1-2CM8, 3CM9-3CM16)

A block diagram of this module is shown in C35260K007. Sixteen modules are provided, and the system is designed for future expansion to a total of thirty-two. The input switch (SW1) is provided on the first set of modules to accommodate the thirty-two outputs of the Optical Receiver modules, and provides some flexibility in interconnections of the system. Mixer MX2 and LO2 upconvert all or a portion of the receiver IF1 bandwidth to the frequency band 8.5-10.35 GHz. The synthesizers which generate LO2 (2G1-2G4, 3G5-3G8) are tunable over a 10.5 to 18 GHz range with 1 kHz resolution. Mixer MX3 and a fixed LO3 10.50 GHz source (2A2, 3A2) downconvert IF2 to the 0.15-2 GHz band, producing the output IF3 signal. Both LO2 and LO3 sources are phase-locked to the 10 MHz reference from the maser. The IF2 bandpass filter (FL1) is designed to provide at least 40 dB of rejection at the MX3 image band of 10.65 to 12.50 GHz. Switch SW2 allows selection of either filtered or unfiltered IF3 outputs for the various backends shown. Output drive level can be adjusted over a 32 dB range with 0.125 dB resolution, via the step attenuator AT1. Power monitoring of individual outputs is not available; however, a square-law detector is provided for monitoring the input power to SW2. The input and output switch states and detected power level are displayed continuously on the front panel. All control of the module is done remotely via the M/C interface.

The up/down conversion process was selected to provide high image rejection and flexible tuning. For a given backend, the user may select any portion of the receiver IF1 band by proper selection of the LO2 frequency. The converter module output signal IF3 is given by:

IF3 = LO3 - LO2 + IF1

where LO3 = 10.50 GHz.

This equation would normally be solved for the value of LO2 needed to convert a particular IF1 frequency to an IF3 frequency needed for a backend. For example, if an IF1 frequency of 3.0 GHz is to be tuned to the 0.75 GHz input center frequency (IF3) of the VLBA Data Acquisition System, LO2 should be set to 12.75 GHz.

As shown in the IF Distribution block diagram (C35260K009), the interconnections between the optical receivers and the converter modules allow up to four independent converters on each polarization of single-feed or dual-feed receivers. (In this diagram the group of labels to the left of each optical receiver indicate which front-end IF1 will appear at each optical receivers' input. This illustration is based on a somewhat arbitrary plan for connections between the front-ends and the receiver room IF Router. A member of the group shown next to each optical receiver is selected by the IF Router setting. The label - L and - R suffixes indicate orthogonal polarizations from a given feed, and the A, B, C, and D differentiate feeds on multi-feed receivers.) For example, CM1-CM4 and CM5-CM8 could simultaneously receive orthogonal polarizations from the 18-22 GHz "A" feed, and four independent portions of the receiver passband could be selected by the LO2 synthesizers G1-G4. Up to two converters may be connected to each IF channel of an eight channel receiver. Because of the high cost of the LO2 synthesizers, only eight are provided initially, connected to pairs of converter modules which will typically receive orthogonal polarizations from a selected feed. As the need arises, and budgets allow, additional LO2 synthesizers may be added.

5.0 Receiver Front-end Frequency Characteristics

Each receiver front-end contains filters or other band-limited components which place constraints on the observable RF input frequency range. Each front-end incorporates one frequency conversion with limits both on the LO input frequency range and on the IF output frequency range. Several of the front-ends provide choices for RF or IF filters, selectable through the M/C system. Table 5-1 gives the frequency conversion data for each front-end.

In order to use a receiver, the LO1 frequency must be set so as to produce an IF1 signal in the proper range. Constraints on IF1 and LO1 settings for each receiver front-end are given in Table 5-1. For most observations, the default value of LO1 should be calculated by tuning the desired sky center frequency to the recommended IF1 center frequency.


Prime Focus Receivers
(Frequencies in MHz)

P.F. Receiver 1

Input Frequency Range:
Band 1: 680 <= Fsky <= 920
Band 2: 510 <= Fsky <= 690
Band 3: 385 <= Fsky <= 520
Band 4: 290 <= Fsky <= 395
IF Filter Select:
960 <= IF1 <= 1200
1040 <= IF1 <= 1120
1060 <= IF1 <= 1100
1070 <= IF1 <= 1090
Recommended IF1 Center Frequency - 1080 MHz
LO1 Range:
       1360 <= LO1 <= 2010

Conversion Formula:
        IF1 = - Fsky + LO1

Gregorian Receivers
(Frequencies in GHz)

1.15-1.73 GHz Receiver

Input Frequency Range:
        1.15 <=Fsky <= 1.75

RF Filter Select:
        FL1 - 1.1 to 1.8 GHz
        FL2 - 1.60 to 1.75 GHz
        FL3 - 1.30 to 1.45 GHz
        FL4 - 1.10 to 1.45 GHz
        FL5 - For future use

IF Range:
        2.6 <=IF1 <=3.4
        Recommended IF1 Center Frequency - 3.00 GHz

LO1 Range:
        4.0 <=LO1 <=5.0

Conversion Formula:
        IF1 = - Fsky + LO1

3.95-5.85 GHz Receiver

Input Frequency Range:
        3.95 <=Fsky <=5.85

RF Filter Select:
        FL1 - 3.95 to 4.65 GHz
        FL2 - 4.55 to 5.25 GHz
        FL3 - 5.15 to 5.85 GHz
        FL4 - 3.9 to 5.9 GHz
        FL5 - For future use

IF Range:
        1 <=IF1 <= 3
        Recommended IF1 Center Frequency - 2.00 GHz

LO1 Range:
        5.95 <= LO1 <= 7.85

Conversion Formula:
        IF1 = - Fsky + LO1

8-10 GHz Receiver

Input Frequency Range:
        7.90 <= Fsky <= 10.10

IF Filter Select:
        1.80 <= IF1 <= 4.20
        2.75 <= IF1 <= 3.25
        Recommended IF1 Center Frequency - 3.00 GHz

LO1 Range:
        10.90 <= LO1 <= 13.10

Conversion Formula:
        IF1 = - Fsky + LO1

12-15.4 GHz Receiver

Input Frequency Range:
        11.80 <= Fsky <= 15.60

IF Filter Select:
        1.25 <= IF1 <= 4.75
        2.75 <= IF1 <= 3.25
        Recommended IF1 Center Frequency - 3.00 GHz

LO1 Range:
        8.80 <= LO1 <= 12.60

Conversion Formula:
        IF1 = Fsky - LO1

18-22 GHz Receiver

Input Frequency Range:
        18.00 <= Fsky <=22.50

IF Range:
        2.00 <= IF1 <= 6.00

Recommended IF1 Center Frequency:
        2.50 for 18.0 <= Fsky <= 19.5
        3.50 for 19.0 <= Fsky <= 20.5
        4.50 for 20.0 <= Fsky <= 21.5
        5.50 for 21.0 <=Fsky <= 22.5

LO1 Range:
        15.50 <= LO1 <= 17.00

Conversion Formula:
        IF1 = Fsky - LO1

22-26.5 GHz Receiver

Input Frequency Range:
        21.80 <= Fsky <= 26.20

IF Range:
        3.50 <=IF1 <=8.00

Recommended IF1 Center Frequency:
        4.00 for 22.0 <= Fsky <= 23.5
        5.00 for 23.0 <= Fsky <= 24.5
        6.00 for 24.0 <= Fsky <= 25.5
        7.00 for 25.0 <= Fsky <= 26.5

LO1 Range:
        17.80 <=LO1 <= 19.20

Conversion Formula:
        F1 = Fsky - LO1

6.0 Backend IF Interfaces

The 1-8 GHz Converter module output ports are connected by coaxial cables to the input ports of the data acquisition backends. The following paragraphs detail these functions.

6.1 GBT Spectrometer IF

Located in the Analog Filter Rack (RNG3 Unit 4) are 24 modules containing bandpass filters which filter IF3 before the signal goes on the GBT spectrometer samplers. These modules also contain total power detectors, the output of which are connected to DCR ports, and so can be used for continuum observations. Descriptions of the two module types follow.

6.1.1 1.6 GHz Sampler Filter Module

The 1.6 GHz Sampler output port (J3) of the 1-8 GHz Converter module is connected to the 1.6 GHz Sampler Filter module (A35208K004). There are eight of these units in the system. This module contains bandpass filters for the 800 MHz and 200 MHz bandwidth modes of the GBT spectrometer. Provision is made for the future addition of two filters, one internal and one external to the module. The coupler (DC1) and detector (D1) provide a means to sense the filter bank output power for adjustment of the digital sampler input level using AT1 in the Converter modules CM1-CM16. Several gain stages boost the output to a level sufficient to drive the high-speed sampler inputs in the Sampler Rack.

In order to observe with these modules, the system LO1 and LO2 synthesizers associated with the front-end and 1-8 GHz Converter module in use should be set to convert the center of the Fsky band to the center of the bandpass filter to be used. The filter bandpasses are:

800 MHz Bandwidth          800-1600 MHz
200 MHz Bandwidth          800-1000 MHz
Stated another way, the input center frequencies of the 1.6 GHz Sampler/Filter modules are:
800 MHz Bandwidth          1200 MHz
200 MHz Bandwidth          900 MHz

6.1.2 100 MHz Converter/Filter Module

One of the four narrowband outputs (J6) from the 1-8 GHz Converter connects to the 100 MHz Converter/Filter module input port. There are sixteen of these modules, one for each 1-8 GHz converter. A block diagram of the module is shown in (C35208K008). The input signal is first filtered to reject the image frequencies, amplified, and mixed with an external 500 MHz reference to produce an output band of 15-350 MHz. The signal is then bandpass-filtered, with selectable bandwidths of 50 or 12.5 MHz. Provision is also made for a third internal filter and an additional external filter, if needed. The signal is then amplified to the level required by the low-speed sampler inputs in the Sampler Rack. A portion of the output is coupled off and fed to a square-law detector to allow monitoring via the M/C interface.

In order to observe with these modules, the system LO1 and LO2 synthesizers associated with the front-end and 1-8 GHz Converter module in use should be set to convert the center of the Fsky band to (500 MHz - the center frequency of the bandpass filter to be used). The filter bandpasses and center frequencies are:

500 MHz Bandwidth          50-100 MHz             75 MHz
12.5 MHz Bandwidth         25.0-37.5 MHz          31.25 MHz
Stated another way, the input center frequencies of the 100 MHz Converter/Filter modules are:
50 MHz Bandwidth           425 MHz
12.5 MHz Bandwidth         468.75 MHz

7.0 Example Setups

As an example, assume a dual-polarization observation with the 12-15.4 GHz front-end (A feed) is to be made, at a sky rest frequency of 15.0 GHz, and using the GBT Spectrometer in 200 MHz mode. The following steps would be done in preparation:

1.  Activate the 12-15.4 GHz front-end. Rotate the turret and deploy the subreflector if necessary.       Select the front-end 2.75-3.25 GHz IF filter.

2.  Set LO1A to the frequency which converts the 15.0 GHz sky frequency to 3.0 GHz; LO1 = 15.0-3.0 = 12.0 GHz. Use the LO Router to connect the LO1A output to the front-end, and set the LO level.

3.  Use the IF Router to connect the front-end A-L and A-R IF signals to Optical Fiber Driver modules; Router input ports 10 and 26 connected to Drivers 2 and 4.

4.  Use Converter modules 2CM1 and 2CM5; connect their inputs to Optical Receiver modules 2 and 4 respectively, using the converter modules SW1 control. Set 2G1 to the frequency which converts the 3.0 GHz IF1 center frequency to 900 MHz, the center of the 200 MHz bandwidth filter:

LO2 = 10.50 - IF3 + IF1
LO2 = 10.50 - 0.9 + 3.0 = 12.60 GHz

5.  Connect the Sampler Filter modules 4SF1 and 4SF5 to 2CM1 and 2CM5. Select the 800-1000 MHz filters, and activate the proper spectrometer modes.