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20th August 2010 05:57
Hello,
hope I'm not telling you guys something you already know :), but pulse-counting usually can't provide good SNR, since for WBFM the amount of frequency change (consider f0=10.7MHz; dev=±75kHz) is very small, after LPF there's little swing. NBFM ~ similar, except for deviation ratio of course. to improve situation (mainly SNR) it's possible to do all sorts of crazy things like multiply 1st IF, then downconvert to Low-IF, build limiters for each IF-stage like some broadcast-band FM tuners (especially Pioneer brand) did, and/or generate multiple pulse trains OR them and run thru adder. but then it's a not a simple detector for casual builder anymore.
the TDA7000 also has that 'mirror image' problem. it somehow rejects image frequency by AFC action, but that degrades the receiver's NF as twice the bandwidth is received. I build a simple dual-conversion CB receiver with 10.7MHz/~12kHz IF. (since I had no decent 455kHz filters lying around.) It pretty much does what it's supposed to, but I had to augment IF selectivity with additional LC tank bw pins 10, 11. something similar would be needed for TDA7088T; I only know ATM that they made some changes in TDA7020T and later to allow really mediocre stereo output in conjuction with a multiplex decoder. I used Sanyo LA1185 IC (=A1185, TA7358, AN7205, ..and others) for downconversion as it has doubly balanced mixer, but generally good results can be obtained even with properly designed self-oscillating mixer. just not so easy to use. :)
27th May 2010 10:54
Alan
Yes, a new work-in-progress web server. I still have the old one at http://sites.google.com/site/arvidevans/ but this new one at http://qrp.webhop.com/ resides on a spare computer (redirected via http://www.dyndns.com) here in the hamshack. Eventually this will include a QRSS receiver and grabber to show your signals on the web server as received here in Meridian, Idaho USA.
Arv
_._
26th May 2010 23:11
Andy,
Indeed, no doubt cost-savings. Gotta wonder though, especially after using a dual channel amplifier. Surely if they are cheaper than single by economies-of-scale then stereo plugs should be by now also?
That said, this radio is clearly a pretty old design, especially as the PCB has positions for 10.7 MHz IFT (that are filled with ceramic filters in this particular implementation). The non-rectilinear mounting of the main IC and the PCB artwork in general is pretty telling too. Squeezing the last bit of life out of a run of old through-the-hole PCBs?
Regards,
Alan
26th May 2010 23:05
Arv,
New website?
The wide-band FM demodulator wouldn't do a very good job SNR-wise with narrow band emissions, at least not without a lot of tweaking. That said I've seen some application notes for using the TDA7088 in narrow-band comms for cordless telephones. You might pick up a cheapy scan/reset FM broadcast receiver from a junk store to source the chip and hack it over for six metres. LO stability would need some thought.
That and the little Kaide radio is a physically nice unit, with not a lot of free-space left inside for hacking. Still, it is pretty cheap if you wanted to buy a few for fiddling with.
One thing I was looking at trying is vaguely related; low-IF pulse-counting detectors. Good selectivity and image rejection are difficult without double-conversion, but a simple few-transistor affair could be thrown together to monitor spot frequencies (like repeater outputs). The low IF should offer reasonable detector output even with narrow-band FM.
Regards,
Alan
26th May 2010 18:57
I suspect the reason why a mono earphone socket is fitted rather than a stereo type is cost. I reckon they save about 0.4c/socket (100,000 up). Multiply by a million or three radios made on the production line, and the savings start to add up. Cost-to-retail markup is roughly x4 so by the time all these little cost savings are added, it makes the difference between a major buyer (like WalMart etc) buying this unit over an alternative.
26th May 2010 05:42
Did you try retuning the Kaide receiver's low FM band to 6 meters to see if it would monitor ham radio repeaters? Might be interesting to try subverting this unit as an ad-hoc 6M monitor to know if the band was open or if your local repeater was in-use. Maybe make it switchable between Sports Ears and 6M Ears?
24th May 2010 10:08
Marxy,
Excellent question, I measured the voltage drop across its limiting resistor to measure it, but I neglected to write that up in the article. It takes about 2.7 mA - surprising little, I'll have to measure in the sports ears unit too.
Regards,
Alan
24th May 2010 09:45
I wonder how much of the Kaide's 17mA current is used by the power LED?
24th January 2014 00:21
dani wrote...