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running dogg wrote:
Alexander Kozik wrote: Hi all, Radio: Tecsun P230. Problem with AM reception (spurious signal?). There is a strong AM station 50 kW with frequency 1530 kHz within 10 miles. Tecsun P230 detects this station when it is tuned to 620 kHz. No other radio detects this station at 620 kHz. Actually, other radios detect weak distant stations at this frequency. Obviously, there is something with heterodyne 1530 - 455*2 = 620 Could you explain what is wrong with radio or just point to the useful web pages that describe the physical/electrical background of this phenomena. Probably, I am not interested how to fix the problem, I just want to understand why it happens. Thanks a lot in advance, -Alex Basically it's due to bad design. All radios have an IF that reduces all incoming signals to a single frequency that then is used for audio output. If you're hearing an internally produced harmonic, then it usually means that your radio is suffering from a design flaw, or that there's some bad parts. Considering how cheaply these Chinese radios are made, it's not surprising that some would have such poor design and/or build quality that internal harmonics or "birdies" would be all over the place. If your radio can't properly handle the IF, then it follows that SW signals will have images too. Such a radio is practically useless for DX. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Assuming the radio is a Tecsun PL-230... It may not be a bad design, it may just be a characteristic of the design. Alex is describing an image rejection problem on the AM broadcast band. In this case it is caused by a very close 50 kW station. He does not describe what his other radios are, so we don't know what it is being compared against. Many AM receivers with a single tuned front end only show 30-40 dB of image rejection. So, with this portable radio, he may be seeing the expected results. Given that the PL-230 is a low end portable, there should be little expectation for good strong signal performance. I would guess that the radio is not broken or flawed. It is probably acting as it should. (Which may or may not be what Alex expected.) ------------ Alex, An image signal is a signal found at 2x the IF frequency away from its proper frequency. The radio has a local ocsillator that mixes with the signals from the antenna. The goal is to change the frequency of the incoming signal to the frequency of the IF where it is filtered and and amplified. The mixing of two signals produces an additional two signals that are at the sum and difference of the two frequencies. The difference frequency is what the IF uses. When tuned to 620 kHz, the oscillator runs at 1075 kHz. The IF is the difference, or 455 kHz. But signals at 1530 kHz also result in the difference being 455 kHz. The mixer converts both 620 kHz and 1530 kHz to the IF frequency. It is up to the radio to filter out the unwanted frequency before it gets to the mixer. If it is successful, then you will only hear the desired 620 kHz signal. However, you have a very strong signal at 1530 kHz. Even if the radio eliminates 99% of the signal before it gets to the mixer, the remaining 1% may be strong enough to hear and may prevent you from hearing a weaker station on 620 kHz. Images can be a problem on any radio. You will see it as a common spec that is frequently measured by manufacturers and reviewers. Methods to reduce the problem include more filtering before the mixer to reduce the unwanted signals, or the use of a higher IF frequency. The higher IF helps because the image is further away from the desired signal so it is easier to filter out. With a high IF, multiple IF frequencies (additional mixers and IF stages) are used to provide the selectivity that is harder to get at the high frequencies. Only you can decide if this is a problem for you. If you see images from AM stations that are much weaker where you are at, then you may have a problem with the radio, but if it is just from very strong station, then you may have to live with it or use a different radio. Hope this helps, craigm |
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