Receiver Design Considerations

Each of the various blocks in the receiver chain has some gain (or loss) associated with it. The receiver chain hence has distributed gain. There are several considerations involved in determining exactly how to distribute the gain across the RF, IF and BB electronics, viz.

1. The response of the entire system must remain linear over a wide range of noise temperatures from cold sky to the high antenna temperatures anticipated when observing strong sources like the Sun.

2. The entire receiver system should remain linear even in the presence of strong interference signals. In particular the inter-modulation distortion (IMD) products should be below a critical threshold^21.1. Also the receiver should have a high desensitization dynamic range^21.2 so that a single dominant out of band interfering signal does not reduce the receiver SNR by saturating the subsystems in the receiver.

3. The RF Front End gain should be such that no more than 1 K noise is added to the Low Noise Amplifier (LNA) input noise temperature by the rest of the receiver chain.

4. The gain should be so distributed that no more than 1% gain compression should occur at any stage of the receiver chain.

5. The level of signals at the input of the cables that run from antenna turret to the base of antenna should be sufficiently high compared to any extraneous interference signals that might be picked by these cables.

6. Components whose contribution to the signal phase needs to be kept constant should preferably be located at the antenna base room where the temperatures are relatively stable compared to that at the prime focus.

7. Internally-generated spurious products (if any) in the receiver, must be very low compared to the receiver noise floor.

8. The Antenna Base Receiver (ABR) input (which receives the the RF signals from the front end through long lengths (about 100 m of cable) should be well matched for the full RF band i.e. 10 MHz to 1600 MHz. A poor match would result in passband ripples.

9. The receiver should have a good image rejection (at least 25 dB). Further since the RF pass band in the common box electronics (see below) has 10 MHz - 2000 MHz coverage, a 70 MHz signal may find a path past the amplifiers and mixer and be coupled into the 70 MHz IF circuitry. The units have to be optimally configured such that a good IF rejection^21.3 is achieved.

10. The ALCs should be active over a large signal amplitude range.

Footnotes

... threshold^21.1

Basically one needs receiver with high enough Compression and Spurious Free Dynamic Range (CDR and SFDR) to handle the range of astronomical signals and interference signals present. In communications receiver parlance, the SFDR is defined as the power ratio between the receiver thermal noise floor and the two tone signal level that will produce third order IMD products equal to the noise floor level. The CDR is defined as the power ratio between the receiver thermal noise floor and the 1 dB compression point. However, for radio astronomical receivers it is customary to define the upper limit for the CDR as the signal level where 1% gain compression occurs and in the case of SFDR, the upper limit as the two tone signal levels which produce IMD products 20 dB below the noise floor.

... range^21.2

The desensitization dynamic range is defined as the power ratio between the level of the strong undesired signal which reduces the SNR by 1 dB and the receiver noise floor.

... rejection^21.3

IF rejection is a measure of attenuation between the receiver input and the IF circuit.