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Can I come in with an idiot's question here? I understand the rationale for one or two TRF stages in a communications receiver, but what is the rationale for several TRF stages in an FM tuner
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Strictly speaking they should be referred as bandpass filter stages and not TRF. Like TRF, the overall bandwidth of the cascade amplifier decreases with increasing number of stages. The attached images are taken from the 1965 IEEE paper written by RCA engineers. It had a double-tuned antenna circuit followed by a single tuned stage. The loaded Q of the double gang is QL=51.4, single gang QL=84.2 and overall Q just under 1Mhz (700-800kHz?) at 3dB. So it is much wider than the FM stereo IF bandwidth of 200kh-300kH. Since each bandpass stage incurs insertion losses, you would need to add more amplifier stages. The goal is not to make the bandpass as high Q as possible, but we want to create steep side skirts of the overall response curve to filter out the spurious signals and images. Beside the front-end RF filter stages, the IF-stages also contributes to significantly to the selectivity of the FM receiver.
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ell, it was the firm's money, for work-work. I'm still intact.
For audio testing, there are programs which use the 'soundcard' of a computer as both source and spectrum analyser. You'll need an attenuator on the source in order to go down to low levels for sensitive inputs. Don't use 'digital' level controls as you will go down into the quantisation noise of the DAC and the computational word-width. The analyser response is also limited by the anti-alias filter, so you don't get to see any higher frequency products... you aren't supposed to be able to hear them, but finding any can give clues to what's going on in the circuitry you're testing.
For RF testing, you're stuck with the expensive stuff. I visit radio rallies and keep an eye out for good quality test gear which is either needing to be fixed, or is too heavy for the trader to want to cart it back at the end of the day.
There is one way round the cost... build a pair of crystal oscillators your chosen separation apart, then your own moderate power amps, combiner and attenuators as needed. This gives you only one test level and spacing but it's a quick one-shot assessment of intermods. I built one years ago into a small diecast box. At an amateur radio show I could feed a test signal into a new model radio being demonstrated. Tune the set to the product frequencies above and below my pair, and see how strong the responses were as a combination of looking at the S meter and by ear. Inevitably someone borrowed this little box and accidentally transmitted into it. End of game. Remarkably few dealers twigged what I was doing, and the ones who found out weren't best plaesed. Obviously they lacked confidence that their product was much good?
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Lucky you. Your work coincides with your hobby, killing two birds with one stone. Work becomes leisure. Pleasure becomes work, work subsiding for the expensive test equipment. The only freebie i have ever got from work is a Hameg 20Mhz oscilloscope that i never use. One day when this hobby will become too expensive and i decide to quite, I may take up trainspotting since i live close to Clapham Junction station. I am sure it is as cathartic as replacing capacitors and resistors. The total expenditure of train spotting as a hobby is the costs of a pen and a note pad. I think it is illegal to use a camera on platforms due to terrorism concerns. So camera is not needed.
I am not clever and experienced enough to build my own IP3 test jig. So i need to forget about IP3 from now
. I have been looking at the SINARD DSP software $67, not bad huh:
https://comtekk.com/sinad.htm
This probably does something similar to a SINARD meter like the link below to measure SINARD, SNR, THD:
https://youtu.be/sBqjNLB6HuM
I am not sure how accurate it is. My junky chinese FM walkie-talkie signal generator can produce output -70--132dm with 800Hz tone. So it can be used be get a ballpark figure of SINARD using the Comtekk.