Choosing a first IF for homebrew receiver - Page 1
Forum section: electronics / rf-microwave

author: ProfessorStank
author_rank: Newbie
timestamp: July 16, 2024, 03:19:32 pm
content: Hi all,As a project to develop my intuition, I'm working on designing a receiver/exciter for the 30m band (10.1MHz to 10.15MHz). I'm going to make it a double conversion superhet with the first IF at a frequency above the receive range, and the second IF way below (probably at 455kHz, The People's IF).Anyways, I'm not sure what I should do for a first IF frequency. I've been thinking about 70MHz, but I wonder if parasitics would make that a pain to work with. Is that something I should worry about, or would I be better served by a lower first IF, like 40MHz or even lower?Thank you

author: ftg
author_rank: Frequent Contributor
timestamp: July 16, 2024, 03:33:38 pm
content: I would consider 21.4MHz or 45MHz as the 1st IF, as crystal filters for those bands are commercially available.If the second IF is 455kHz, then one is in luck, as the suitable crystals for mixing 21.4MHz or 45MHz down to 455kHz are also widely available.If you are making something SSB capable, then your second IF kainda gets decided by the SSB or CW filter can build or get.A nice collins filter 455kHz will work very well, but cost quite a bit.Or something can be diyed with crystals around 4.194304 - 4.43361875 - 4.9152 for example.As those are common and others have used them as well to build SSB and CW filters, so there would be implementations by others to compare against.21.4MHz might a bit marginal due to 2x 10.15 being so close to it, the frontend would have to be quite tight and robust to ensure that strong stations don't cause unwanted signals to appear on the 455kHz IF.Edit: fix'd a typo.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 16, 2024, 03:52:42 pm
content: There are 70 MHz SAW filters available, don't know if they would be adequate?About $20 ebay.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: July 19, 2024, 10:03:42 am
content: Quote from: ProfessorStank on July 16, 2024, 03:19:32 pmThe conventional transceiver manufacturers have mostly abandoned the configuration of a VHF 1st IF followed by a lower one in their higher spec transceivers. Not only does it complicate matters, but the performance is degraded by (a) using a wide VHF filter and (b) you add noise by using an additional mixer, oscillator and having to overcome the loss of two filters (as opposed to just one in a single conversion). The reason for the high first IF was to have high image to signal rejection ratios and to give 10 KHz to 30 MHz coverage without switching IF frequencies, and without gaps around an 8 or 9 MHz IF frequency. For ham band only use, an 8 or 9 MHz single conversion IF receiver is fine, just pay attention to the front end band pass filter, you can easily obtain 70 dB image rejection.The Kenwood TS-890, Elecraft K3s and the Yaesu FTDX101 are all single conversion with a very high specification.My home brew receiver uses an 8.215 MHz IF using surplus Yaesu filters. Details including circuits and board layouts at:https://www.qsl.net/g4aon/g4aon_rx/73 SJ

author: radiolistener
author_rank: Super Contributor
timestamp: July 19, 2024, 10:13:51 am
content: From my experiments, too high IF is vulnerable for parasite reception on IF frequency and needs proper shielding and good filters.At 45 MHz I had too high sensitivity for parasite reception near IF frequency.I think 10 MHz is optimal value for IF.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: July 22, 2024, 06:43:29 am
content: Quote from: Solder_Junkie on July 19, 2024, 10:03:42 amI tend to think along the same lines. For a single band 30m CW rig, single conversion is all it needs. Mostly my go to xtals for making filters for single conversion are the 4.123456mhz or whatever the exact value is type ones and its rather easy to make a cohn min loss CW filter with a couple hundred Hz bandwidth. And using digital synthesis for the BFO you can put the signal on either side of the filter easily and not worry about inversion with SSB. It just makes life simple.