[Elecraft] K3 IF Design

[David Woolley (E.L)]

ab2tc wrote:
> 
> Somehow the math here is wrong. With a 1st IF of 8.215MHz and the local
> oscillator at 22.215MHz in order to receive 14.000MHz, the image frequency
> is 8.215+22.215 = 30.430MHz. Either way, it 's not hard to achieve excellent
> image rejection across the HF band with this IF frequency. Icomwood would do

I believe the history of up conversion receivers started with the Wadley 
loop design, which allows selection of the MHz part of the frequency 
without using PLL or digital techniques.  It is a triple conversion 
design.  The first local oscillator mixes up to a 1MHz range at about 
42MHz.  The first IF has a 1MHz bandpass filter.

The second local oscillator is derived from the first by mising it with 
a harmonic rich crystal source and then using a very narrow band filter 
to select just one of the mixer products.  The overall result is that 
1MHz of spectrum is shifted to an IF of about 2.5MHz with a frequency 
accuracy that only depends on the crystal, but using only discrete 
analogue circuitry.  A conventional supehet then tunes across the second 
IF to interpolate between the 1MHz points.  The ultimate frequency 
stability was that of an analogue VFO receiver covering, say, 2 to 3 MHz.

The Wadley design receiver I have (FRG 7) uses a manually tuned 
pre-selector at the input frequency, but when people started developing 
fully synthesized full shortwave coverage, receivers, they wanted a 
design that didn't require the user to have to select a pre-selector 
range and tune it properly, or to have to maintain calibration curves to 
enable one to be electronically tuned.

For the more consumer  products, like the Sangean 803A (sold under other 
brand names by Radio Shack), I believe they basically just used a 30MHz 
low pass filter.  For the amateur radio market, they used a small number 
of band pass filters, typically each with a 2:1 frequency range.  Having 
an image at around 70MHz was good for this sort of design.

These designs did use synthesizers, so they used a narrow filter at the 
42MHz, rather than a 1MHz one and relied on the synthesizers to to 
create a stable frequency with small steps.

Receivers like the K2 and K3, whilst they use a band pass filter rather 
than a manually tuned pre-selector, use filters that cover one, or maybe 
  two amateur bands.  That means a lot more filters.  The K3 with 
general coverage filters, outside of an amateur band, uses a 
pre-selector system more like the up conversion receivers, but probably 
isn't intended to provide ultimate image rejection outside of the 
amateur bands.

(The pres-selector is probably advantageous for the Wadley designs as 
the VHF pass band filter is a (multisstage) LC one and has to have a 
pass band of more than 1MHz, and the filter that selects the particular 
second local oscillator frequency will leak some at multiples of 1MHz 
away, so one probably needs to combine pre-selector, VHF passband and 
second local oscillator filters to get really good rejection of images 
at multiples of 1MHz.)

> better arguing that the IF rejection will be poor when receiving close to
> the IF frequency, but with a trap tuned the IF, the K3 does OK even though
> there is a hole in coverage close to the IF frequency. Another disadvantage
> of the low IF is that the synthesizer must cover a much larger range
> relatively speaking, but it appears that the K3 synthesizer performs
> admirably in spite of this challenge. The advantage of being able to offer
> excellent filters at the 1st IF is undisputable.


-- 
David Woolley
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