K3 IF Design
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Shane
In the case of 14 MHz and the K3, the first IF is 8.215 MHz. An oscillator at 22.215 MHz mixes with 14 MHz to give the difference frequency of 8.215 MHz, however there is another difference of 14 MHz minus 8.215 MHz which is 5.785 MHz which is the image frequency. The ratio in dB between the wanted frequency and the image gives the image rejection. There are other images involving the 2nd IF of 15 KHz too. With the K3, the main images are greater than 70 dB below the wanted signal. My own measurements give figures of 70 dB on 28.1 MHz (worst case) to better than 110 dB on 1.8 MHz and 50 MHz. The big advantage of a low 1st IF is the ability to use narrow "roofing" filters to reject nearby interference. The disadvantage is potential for poorer image rejection. Image rejection is only one aspect of a transceiver design, of model A was better in all respects than model B (and was cheaper) everyone would buy model A, but life isn't so simple! 73 Dave, G4AON http://www.astromag.co.uk/k3/ ---------------------------------------- Does the K3 manage to obtain image rejection comparable to Icom's and Yaesu's flagships? If so then it seems to me that Icom and Yaesu seem to be relying on a proven RF design and instead are concentrating their current efforts on ergonomics and presentation. There's nothing wrong with that though. Thanks, Shane VK5ABQ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Thanks Dave. I'll read your review.
Having been out of HAM radio for over a decade, I'm just trying to understand where each company involved in selling radios is heading and what their priorities are.
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Hi,
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 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. Knut - AB2TC
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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 "The Elecraft list is a forum for the discussion of topics related to Elecraft products and more general topics related ham radio" List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm> _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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