[Boatanchors] Question About Receiver Design - Crystal Controlled Converters

[Richard Knoppow]

    It should be pointed out that images are not the only 
spurious responses,  multiple conversions tend to generate many 
spurious responses due to higher order images, beating of the 
various oscillators, etc. Design books will detail them. The 
design becomes more difficult if one wants to eliminate or reduce 
them as much as possible. Because it is possible in modern design 
to get a high degree of selectivity with high frequency filters 
the idea now is to go back to a single conversion but have the IF 
higher than the highest frequency the receiver is designed for. 
That is, to use up conversion for all frequencies.
     Collins reduced the number of crystals used in the 51J and 
other receivers by using both up and down conversion, in this 
case meaning the first conversion had the first conversion 
oscillator above the signal for some bands and below for others, 
and using crystal harmonics for some bands. The 51J is 
interesting because it uses double conversion for most bands but 
single conversion for two and triple conversion for the broadcast 
band to avoid getting signals into the tunable IF. The single 
conversion bands use the tunable IF directly, one with it above 
the signal frequency and one with it below. Very clever but very 
complex. The 51J is generally a clean receiver but does have some 
spurs.
    Goodman's design eliminates some of these problems by being 
more complex.
     The only receivers without conversion spurs are TRF sets.

On 8/11/2022 7:54 AM, Al Klase wrote:
> Scott,
> 
> Important parameters in any receiver are frequency accuracy, 
> stability, and freedom from spurious responses (image rejection).
> 
> It's pretty easy to design an 80-meter set with, for example a 
> 455 Khz IF, that offers good performance.  However, as we go up 
> in frequency everything become more difficult.
> 
> Multiple conversion (double in this case) reduces the image 
> rejection problem.  The image will be two times the IF frequency 
> away from the desired signal.  So 910 KHz in the case of 455 IF, 
> becomes about 8 MHz with a nominal 4 MHz first IF.  So the 
> preselection filtering is easier to accomplish.
> 
> Crystal control of the first oscillator results in essentially 
> the same frequency accuracy and stability as you's have on 80 
> meters, even when operating on 10 meters.
> 
> Down side is the cost of the crystals, but if you're only worried 
> about a couple of bands, it's not too bad.  It's a much bigger 
> problem for general-coverage radios.  Take a look at the Collins 
> sets like the 51J and R-390.
> 
> Hope that helps,
> Al
> 
> 
> ARK Sig Block Al Klase - N3FRQ
> Jersey City, NJ
> http://www.skywaves.ar88.net/
> On 8/11/2022 9:08 AM, whitebear1122 at comcast.net wrote:
>> I'm looking at Byron Goodman's HB67 receiver in the ARRL 
>> Handbook 1965.  His
>> design uses crystal controlled converters for 40-10 ahead of an 
>> 80 meter
>> receiver.  I see this is single conversion on 80 meters as it 
>> bypasses the
>> converters, then double conversion on 40-10.
>>
>>
>> What are the advantages and disadvantages of this configuration 
>> over the
>> other receiver designs where the local oscillator is changed to 
>> mix 80-10?
>>
>>
>> Thanks. 73 Scott WA9WFA
>>
>>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL