Electronics > RF, Microwave, Ham Radio

Order / Gain Structure of RX Front End?

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elmood:
Hey folks,

I'm working on a homebrew design for an HF transceiver and there seem to be just as many approaches to receiver design as there are radios out there.

Are there any best-practice rules on the order of blocks / gain settings for receiver front ends? What is the normal thought process that goes into deciding how much gain to put right after the antenna, after filters and attenuators, after the IF mixer, etc.?

I'm working on a basic block diagram for my project and feel funny just trying to arbitrarily stick things together.

73 VE2EOT

T3sl4co1l:
Doesn't have to be any; the downside is mixer noise/loss is in full view.  Usually a preamp is added to deal with that, and that's about it.  So, if typical mixer loss is say 6dB, adding a preamp somewhat over 6dB, and with better than 6dB noise figure, will improve sensitivity.  Note it should have high IP3 to avoid mixing/clipping from strong (local) signals.  Or can be looped in to AGC if that helps to a similar end.

Preamp also affords isolation, in case the mixer is prone to spurious outputs from the RF port.  So, a good idea for a single diode mixer (but, you'd probably only use one of those at frequencies where you can't amplify in the first place, i.e. vintage microwave hardware?).

Isolation can also improve filtering, as antennas are a mess of reactances.

Tim

A.Z.:
I agree with teslacoil, a simple yet quite effective design is to place a wideband and low gain stage at the antenna input, a simple JFET common gate stage will work well there and give both impedance adaptation and I/O isolation, following such stage you'll have a bandpass filter cutting out the undesired frequencies and, at the output of the bandpass, a preamp wich will offer the desired gain and that may be AGC controlled, after that ... well it depends from the design you'll use for your receiver

my 2 cents

radiolistener:
There is a book which explains what you're looking for:


Joy Laskar, Babak Matinpour, Sudipto Chakraborty
Modern Receiver Front-Ends: Systems, Circuits, and Integration

rfdave#gmail.com:
Generally, all this is worked out in a spreadsheet that calculates all of this. I haven't done this at HF yet, just VHF and higher, so the noise/linearity tradeoff is significantly different. ICOM published a "IC-7850/IC7851 Technical Reference Book" that goes into the details of this radio, including a nie detailed block diagram.
Also, "Modern Communications Receiver Design" by Drentea might be a more relevant reference.

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