2m Receiver Design

[NZ0I]

This topic is for discussion related to the design and implementation of a 2m ARDF receiver using the Receiver Development Platform.

[NZ0I]

Once we have a better handle on the gain to expect from the SA605, we can fill in some of the question marks in the earlier stages. The three clocks shown will be provided by the three CLK outputs from the Si5351. Additional attenuation needs to be added early in the receive chain. Attenuating the VFO signal directly will probably not work very well if we use the 

LT5512 mixer, since the LO buffer of the LT5512 keeps the conversion gain constant over a wide range of LO drive power levels. Making the mixer post-amp variable gain would probably be a better choice.

[NZ0I]

Based on the SA605 specification, we might expect to realize the following gain values from the SA605 in our configuration:

Input mixer: ~+10dB (between pins 1 and 20)

IF Amplifier: 0dB (not used in our configuration - to avoid instability/limiting)

IF Filter (xtal insertion loss): ~-2dB (between pin 20 and output of xtal filter)

Gain-killing Resistor: ~-20dB (across R410 - this is really just a guess - we need to adjust this for maximum gain short of causing instability or limiting in the Limiting Amp.)

Limiter Amplifier: <+60dB (this is likely to be lower than spec due to using a higher IF frequency than recommended)

Output mixer (quad detector): ~+10dB (assumed, since it it also a Gilbert cell detector like the input mixer)

Total Gain Estimate: 50dB to 70dB

This is a hand-wavy estimation. Much uncertainty is due to using the SA605 in an unconventional manner to achieve a superhet receiver.

If we measure total gain near or below 50dB, we should experiment with lowering the value of the gain-killing resistor. If we measure total gain near the upper end of this estimate then we're probably getting as much out of the SA605 as we should expect... but we can still try tweaking values to see if we can do any better.