Receiving the IF frequency of a receiver - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: September 27, 2019, 04:11:21 pm
content: Well, after finding a bunch of old radio band selector switches ,the thought about building a multiband receiver for HF (from 1Mhz upto 30MHz) uoccurred to my mind. In order to get a good image rejection I thought of following a dual conversion superhet design. I took 10 MHz and 455KHz as the first and second IF respectively (Just because I already had all the necessary components in my stock)Here comes the problem,Would I be able to listen to the 10 MHz part of the HF band? Might sound silly, but what I'm unable to make out is that 'Appearantly for 0 Hz LO, I'll get 10 MHz output from the first mixer.". So how is it possible to listen to that part of the band?

author: fourfathom
author_rank: Super Contributor
timestamp: September 27, 2019, 04:36:21 pm
content: Quote from: Dom13c on September 27, 2019, 04:11:21 pmWith a 10 MHz first IF, you will have a hard time tuning near 10 MHz. If you are using a diode mixer, a 0 MHz LO is just a DC current. That could work and can indeed receive a 10 MHz signal. But say you want to tune to 10.14 Mhz (in the ham band). You will need a 140 KHz LO, and you will have an image frequency of 9.86 MHz. There is no practical way (short of DSP) to filter out that image. This may or may not be a problem, but we generally don't like to have poor image rejection.And remember, with a 10 MHz first IF, you will get sideband inversion as you tune below and above 10 MHz. This isn't a problem, but you need to be aware of it.This is why modern wide-tuning single and dual-IF receivers usually have a high first IF frequency, well above the top of the desired frequency range.

author: Dom13c
author_rank: Regular Contributor
timestamp: September 27, 2019, 06:30:51 pm
content: I was planning to use an all analog VFO cause I don't want to put a squre wave, like the si5351 generates ,in the mixer asit increases the noise in the audio output . So couldn't shift the first IF to a much higher level like the Yeasu ft817 does(at 68 MHz) just because at that high frequency vfo drift would be too much to nicely copy ssb signals . Do you think that using two seperate crystal filter for different bands is something to fix the problem of receiving 10 Mhz up?

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 27, 2019, 06:45:37 pm
content: If you want to receive 1-30MHz you have 3 options:- A low IF like 455KHz but that will give problems with mirrors and you will need a very high Q frontend- A high IF like 45MHz but as you said that will need a very stable VFO.- A barlow wadley kind of system which cancels the drift of the VFO but is not that easy to build.Receiving at the IF frequency will never work in a single conversion system.

author: fourfathom
author_rank: Super Contributor
timestamp: September 27, 2019, 07:08:57 pm
content: Quote from: Dom13c on September 27, 2019, 06:30:51 pmActually, a squarewave is close to a perfect waveform for many mixers, as long as your front-end filter takes care of the possible harmonic mixing products. The si5351 can be pretty clean, but there are indeed annoying spurs with some divide ratios. How annoying depends on your goals.Building an analog VFO that mixes the full HF band to a 10 MHz IF is going to be challenging because of the huge frequency range. This is another reason why high-IF designs are so popular.You might look at the ARRL handbook (especially older editions) to see how the low-IF dual-conversion designs handle these issues.

author: Dom13c
author_rank: Regular Contributor
timestamp: September 28, 2019, 04:37:59 am
content: Quote from: PA0PBZ on September 27, 2019, 06:45:37 pmWhat would you suggest as the mixer topology(e.g. double balanced diode ring mixer, Polyakov mixer, or dual gate mosfet mixer) even if I try my hands on the barley-wadley type vfo. Saw them a while back on YouTube (w2aev), but their complexity along with the lack of a frequency counter discouraged me to tinker with them. But seems like I ain't got a choice but to make such a vfo to get what I want.

author: fourfathom
author_rank: Super Contributor
timestamp: September 28, 2019, 06:03:52 am
content: Quote from: Dom13c on September 28, 2019, 04:37:59 amThap Polyakov mixer is interesting, but it does look like it's going to inject a lot of the LO back into the RF input port. If you don't have a high reverse-isolation amplifier feeding the mixer you could generate some serious interference.For an extremely wide-range and stable LO, you should consider a good Direct Digital Synthesis chip (AKA "NCO"). These also will have spurs, but with a wide-enough D-to-A converter the spurs can be minimized to virtually nothing.

author: bob91343
author_rank: Super Contributor
timestamp: September 28, 2019, 06:45:51 am
content: What is often done is to make the first oscillator a crystal fixed frequency and thus have a variable first IF. Then you use a simple superhet to cover this new range.That way the stability is good.

author: szoftveres
author_rank: Regular Contributor
timestamp: October 03, 2019, 10:39:39 pm
content: QuoteIn general it's a very bad idea to design a receiver where the received band covers any of the internal frequencies (IF for example). If you're looking for double conversion on a receiver that covers 1Mhz up to 30MHz, the best practice is to up-convert first (e.g. to TV IF frequencies, TV IF transformers are somewhere in the ~40MHz range) for the best image rejection, then down-convert (e.g. to 455kHz) for selectivity.By up-converting first to e.g. 45MHz, the input filter can simply be just low-pass filter with a ~30MHz corner frequency.Note: a tunable LC tank would be much better for the input, but it's very difficult to design one for such a wide range with proper LO tracking.