In the design and implementation of an RF receiver, I'd like to build and verify it stage by stage, with particular attention to impedance matching, noise figure and gain.

If I had access to a professional RF lab, I'd be using network analyzers, VSWR meters, spectrum analyzers, an RF-anechoic chamber and a reference transmitter, but I have none of those things, nor the desire to burn a few hundred thousand dollars to get them.

I'd like to plan a methodology that can give me some assurance that the receiver I make is going to work, and give me some idea of the minimum receivable signal. This is far before the stage where I'm able to say what exact gain, noise figure, selectivity, etc. the receiver will need. If it picks up anything at all, I'd be happy (for the first iteration).

Let's assume that I have a basic DMM, and a function generator and scope that are both capable of 3-15 MHz, the band the receiver will use.

Perhaps a methodology could be:

  1. Simulate the antenna and tuning network to verify that they are expected to work. My concern here is that antenna predicted vs. actual impedance will probably vary. The formula I have does not account for inter-winding capacitance or wire resistance.
  2. Simulate again, but add the equivalent circuit of an oscilloscope and lead+probe. This is expected to add capacitance (lower resonance frequency) and resistance (lower Q).
  3. Build the antenna (in this case a manually wound AM air core loop antenna)
  4. Connect the oscilloscope to the antenna
  5. Place generator cable around core of antenna for inductive coupling
  6. Short leads of generator cable to each other
  7. Set the generator frequency to 3.33 MHz, the first station I'm interested in receiving
  8. Starting at 0, increase amplitude of generator until signal is seen on scope
  9. Add tuning cap to antenna leads
  10. Verify that signal as seen on oscilloscope peaks when cap is about in centre of designed adjustment range
  11. If previous step shows a heavily lopsided response, i.e. tuning cap is unable to get antenna to resonate, adjust generator frequency until resonance is seen, and calculate backwards to infer antenna impedance and required tuning network

For a budget/hobbyist design, does this methodology make sense?

  • \$\begingroup\$ The Tuned input and Local oscillator must track across the entire band. Start the alignment at the highest end of the highest band, and adjust down to the lowest frequency of the lowest band. Do this several times until you are sufficiently satisfied and or tired. There's much more work if you have a RF stage ahead of the mixer. Consider a Noise Bridge with the Scope. \$\endgroup\$ – Optionparty May 27 '18 at 1:54
  • \$\begingroup\$ @Optionparty I hadn't considered the LO yet. That would be tackled later. Also, you seem to be describing a receiver for continuous channel reception within a wide band, but my situation will be slightly different. There are only three preset channels. \$\endgroup\$ – Reinderien May 27 '18 at 8:32
  • \$\begingroup\$ Before you do any of that you simulate. You also don't put your generator cable through the core as you appear to be describing. You place it around the core. \$\endgroup\$ – Andy aka May 27 '18 at 10:02
  • \$\begingroup\$ @Andyaka Noted, edited. \$\endgroup\$ – Reinderien May 27 '18 at 22:12
  • \$\begingroup\$ Be aware the scope has 10pF or so of input capacitance, plus some input ESD protection resistance that will dampen the resonance. \$\endgroup\$ – analogsystemsrf May 28 '18 at 4:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.