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Skywave · 23rd Jul 2008, 12:37 am

Dominic: May I be a little bit blunt here, without giving offence?

Re: your Q. above . . . in a word . . . no. It isn't worth the hassle.

The only reason for going down the dual-conversion route on a comms. RX is to get good selectivity by virtue of a low IF (which must be the 2nd. IF) and, at the same time, good image rejection - by virtue of a high 1st. IF.
Typical figures for fixed IFs with this arrangement are: 1st. IF in the range 1.6 to 1.8 MHz; 2nd. IF 50 kHz to 100 kHz.

Agreed, 11 MHz will meet the criterion of a "high 1st. IF" and, therefore, is a mathematically sound approach. But the gain per stage compared with a 455 kHz IF will be a lot lower. You'll need several stages of amplification at 11 MHz using double-wound IFTs to get about the 80dB (or thereabouts) of gain that will be needed. (This addresses the point raised by Sean above. I'm tempted to delve into the relevant gain calculations over all this, but I'll leave that - for those interested - to refer to the classic works by Langford-Smith, F.E. Terman, M.G. Scroggie, Everett & Anner et alia.)

However, with regard to the Q. raised immediately above this Post, the really relevant Q. that has to be answered first is:
for a comms. RX for the 80m. AmRad band (3.5 - 3.8 MHz), is a dual-conversion RX necessary? To which, IMHO, the answer is no; it is simply over-kill for that freq. band.

Classic comms. receivers - when operating on 80m. - for the most part amount to straighforward superhets with one* tuned RF stage (pre-mixer). Either single-conversion to 455 kHz or double-conversion* (1.6 Mhz / 85 kHz). The more exotic ones with tunable IFs are a possibility for consideration - but that moves this Topic away from the 11 MHz filter - which is at the heart of this Thread.

* RXs that employ two RF amps. and / or double-conversion also cover much higher frequencies. Hence the two RF and/or 2x conversion: needed for input RF above 13 MHz (approx).

Mike: I don't think you'll go far wrong if you simply follow some well-tried routes that the designers of these aforesaid receivers have followed. Some of the Eddystone receivers spring to mind as examples worthy of study.

Yes, I do understand that your 11 MHz filter is a tempting beast that is simply begging for use - I have several such 'animals' here myself - but the blunt fact is that it is simply an inappropriate component for your particular application.

(A couple of random ideas: depending on its bandwidth, a better use could be as a a filter-block between a 144-146 MHz fixed-tuned converter feeding a comms receiver acting as a tunable IF, tuning 10 - 12 MHz. Or possibly in an FM tuner - IF strip).

I am sincerely regretful if all that lot sounds a little bit hard - but I am trying to save you from what could be a lot of wasted effort.

Regards,

AL / Skywave.

23rd Jul 2008, 12:37 am	#9
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: 80 meter superhet Dominic: May I be a little bit blunt here, without giving offence? Re: your Q. above . . . in a word . . . no. It isn't worth the hassle. The only reason for going down the dual-conversion route on a comms. RX is to get good selectivity by virtue of a low IF (which must be the 2nd. IF) and, at the same time, good image rejection - by virtue of a high 1st. IF. Typical figures for fixed IFs with this arrangement are: 1st. IF in the range 1.6 to 1.8 MHz; 2nd. IF 50 kHz to 100 kHz. Agreed, 11 MHz will meet the criterion of a "high 1st. IF" and, therefore, is a mathematically sound approach. But the gain per stage compared with a 455 kHz IF will be a lot lower. You'll need several stages of amplification at 11 MHz using double-wound IFTs to get about the 80dB (or thereabouts) of gain that will be needed. (This addresses the point raised by Sean above. I'm tempted to delve into the relevant gain calculations over all this, but I'll leave that - for those interested - to refer to the classic works by Langford-Smith, F.E. Terman, M.G. Scroggie, Everett & Anner et alia.) However, with regard to the Q. raised immediately above this Post, the really relevant Q. that has to be answered first is: for a comms. RX for the 80m. AmRad band (3.5 - 3.8 MHz), is a dual-conversion RX necessary? To which, IMHO, the answer is no; it is simply over-kill for that freq. band. Classic comms. receivers - when operating on 80m. - for the most part amount to straighforward superhets with one* tuned RF stage (pre-mixer). Either single-conversion to 455 kHz or double-conversion* (1.6 Mhz / 85 kHz). The more exotic ones with tunable IFs are a possibility for consideration - but that moves this Topic away from the 11 MHz filter - which is at the heart of this Thread. * RXs that employ two RF amps. and / or double-conversion also cover much higher frequencies. Hence the two RF and/or 2x conversion: needed for input RF above 13 MHz (approx). Mike: I don't think you'll go far wrong if you simply follow some well-tried routes that the designers of these aforesaid receivers have followed. Some of the Eddystone receivers spring to mind as examples worthy of study. Yes, I do understand that your 11 MHz filter is a tempting beast that is simply begging for use - I have several such 'animals' here myself - but the blunt fact is that it is simply an inappropriate component for your particular application. (A couple of random ideas: depending on its bandwidth, a better use could be as a a filter-block between a 144-146 MHz fixed-tuned converter feeding a comms receiver acting as a tunable IF, tuning 10 - 12 MHz. Or possibly in an FM tuner - IF strip). I am sincerely regretful if all that lot sounds a little bit hard - but I am trying to save you from what could be a lot of wasted effort. Regards, AL / Skywave. Last edited by Skywave; 23rd Jul 2008 at 12:56 am. Reason: General tidy-up; a complicated Post!