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On Jan 31, 1:51 pm, Owen Duffy wrote:
K7ITM wrote in news:9e844e58-a673-4ec0-9a0b-ec15f8cc8f30 @c4g2000hsg.googlegroups.com: On Jan 31, 12:31 pm, Cecil Moore wrote: K7ITM wrote: To me, having a linear power scale is a big advantage, because then you can reasonably accurately figure SWR without having to worry about temperature compensation of the detectors. Can you define what you mean by linear? Straight line? Since we can only measure voltage and current, in order to obtain a linear power scale from a linear meter, it is necessary to supply some pre-display computing ability (microcomputer). -- 73, Cecil http://www.w5dxp.com See earlier posting in this thread. See various Avago ap notes, such as AN 969. A diode detector run at low input provides an output DC voltage that's a constant times the square of the input RF voltage. If the input voltage is, or is assumed to be, at some constant resistive load impedance, the DC output is linear with RF power input. The proportionality is temperature dependent, but if two detectors are constructed the same and run at the same temperature, and run in the signal level region where that relationship holds, then the ratio of the output DC voltages is a very good approximation of the ratio of the input RF power levels, and thus is useful for finding the SWR if the detectors are attached to the forward and reverse ports of a good directional coupler. Top end of the useful "linear power" range using an HSMS-2850 single diode detector is about 10mV DC output. If you can measure the DC accurately down to 1uV (a bit tough, given thermal emfs, but possible), that gives you about a 10000:1 power range, or 100:1 RF input voltage range -- or about 1.02:1 SWR. Chances are very good that a home-built coupler won't be accurately enough matched to 50+j0 ohms to worry about anything that low anyway, even if you had a reason to care about it. Cheers, Tom Tom, This is further from Suzy's needs, but... Operation of a diode detector in the square law region isn't out of the question, but it takes some serious gain to drive a meter. There are some good chopper stabilised op amps out there that have uV offset levels and single supply rail and input to below the negative rail eg LTC1050. Another alternative is the AD8307AN log amps for a linear dBW scale. You could even use one on FWD and REF detectors and difference the outputs in an op amp for a direct indicating VSWR or RL scale. I have thought of getting one of these chips and seeing whether its response is fast enough to drive a PEP amplifier for SSB telephony. Back to Suzy's problem... The instrument downstream of the sampler is not so much the issue as building and calibrating a sampler when you have no test gear. Suzy, if you see a Revex W560 going on VKHAM for $100 or so, it is a good buy. It has HF to 70cm (two independent couplers, ie four coax connectors), and works pretty well. For a dummy load, the market was flooded with terminations from 25W to about 60W that had been scrapped from AMPS base station equipment, and they were sold at hamfests for $20 or so, you may find them if you look around. Owen Yes, there are several linear-in-dB RF detectors out there. Linear Technology also have them. I really like that idea; they're typically much more temperature stable than a diode detector. But Suzy wanted to avoid SMT. I've used a Harris chopper-stabilized op amp with HSMS-2850 zero-bias detector diodes, and it works well, but I did learn something about the need to be really careful around the chopper capacitor pins on that op amp before getting it right... But it's also not difficult to find a digital voltmeter that will go down to pretty low voltage at high impedance. A 4.5 digit meter on a 200mV scale does ten microvolts, and the simulation I ran last night suggests you could see down to about -50dBm power level with that. When you get down to 10uV, you have to get serious about avoiding thermal emfs. I suppose it makes sense to just drive the detector hard and run it right into an analog meter movement, and then calibrate the meter. Actually, at that level, the detector should be pretty linear in voltage. That actually makes it easier to detect down closer to 1:1 SWR anyway. I posted not too long ago about a load I made with four 200 ohm 2 watt metal oxide resistors that shows what to me is remarkably good return loss out to well beyond 450MHz. It was very cheap to make. But there's no guarantee that some other brand of resistor would give such good results. It may have just been a fluke that the one I made turned out so good. (But I'm not about to toss it out!) Cheers, Tom |
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