AN920

I am trying to say that very little DC offset on the input will make the peaks uneven. Read your generators instruction manual to find out where is the cal position. Some generators will have a internal adjustment to calibrate the DC offset zero position. That may be out of calibration. More expensive digital synthesized generators will be auto cal'd by the uP and offset will be very close (within uV's) to zero. Just checked my digital generator and the offset is 11uV. I think this will be equal to 1LSB of the internal DAC

Your function generator may have a dirty/noisy offset pot. You don't need much offset to mess up the result. Have you tried making R3 variable and adjust for balance?

I managed to find a model for the BFR90A. It turns out that it can perform a bit better. More gain overall, but with the previous bias the input match above 1GHz was only -5dB. Running the transistor at 10mA instead of 7mA improved this condition. First plot shows the problem with bias 7mA Last plot shows the improvement at 10mA

OK, here is the modified input circuit to try. The first picture shows the original circuit and a plot of the gain (blue) and input match (red). It is clear that we have loads of gain (23dB) from near DC. As bugs will normally be in the FM band (88-108MHz) and up, we don't want this gain at the low end to pick up PC psu's, tube lights etc. The red trace shows that the input match (50 Ohm) improves with higher frequencies. At 2GHz the return loss is about -12dB The last picture shows the modified schematic. Notice that below about 30MHz we have a rapid gain drop-off. Input match is very bad resulting in high attenuation below 30MHz. Above 30MHz we can see the gain increasing and peak around 500MHz. Also note that the input match gets much better over the frequency range. The best possible with the old circuit was -11dB. Now we are below -13dB after about 150MHz with -17dB at 350MHz. The BFR93A model that I used should be close enough to the BFR90/1A Ideally we need more gain, something near 20dB+ to make the circuit more sensitive at the upper end. Another alternative is to use a simple gain-block that will have enough gain up to 2GHz. That together with zero bias schottky RF detector diodes as a voltage doubler should be sensitive enough.

The circuit need a much better high pass filter on the input, to eliminate the low frequency noise that is triggering your circuit. The RC filter is not enough. Another way will be to have a switchable filter for commercial FM band, mobile etc. The circuit also need to me in a metal or screened box so the only signal comes in via the antenna. You can paste tin foil on the inside of your plastic box to screen this. I will look at designing a suitable filter/s that you can try.

Yes I have seen a few variations on this circuit function!

I tested his circuit as well. Worked well after R3 was adjusted for balance.

I constructed his circuit and made R3 variable and obtained good results. I also modified the circuit as shown and waveforms looked good up to 1kHz+ Results agreed very much with the simulations.

I think that his problem is due to resistor tolerances. Doing the math on his circuit shows that using 1% resistors the peaks can differ by as much as 10% One of the resistances (R3) should be made variable to adjust for proper balance. Using 5% resistors, peaks can differ by 50%

Start a process of fault finding. You can't just rely on the calculations. Check circuit connections. Increase the resistance values to see any improvement. Change the opamp or use another type. Make sure your AC signal you feeding in does not have any DC offset. This will cause what you are seeing. This can happen when you are using a function generator with the offset control not zero.

His 5V p-p signal should not cause any problems at this minimum current spec.

Simulation also show no obvious problems using the part values in his diagram.

Should work. Re-check all your wiring for a possible mistake

Many cheap bugging devices work in the commercial FM band so that may cause problems detecting them out of all the other stations.

The circuit have different clock settings for fast and ultra fast setting as well. There is one setting for normal run mode that uses the 1Hz output. For calibrating the crystal frequency a frequency counter will be handy. The 4040 is just dividers. You don't need to have all the settings. Just run, set and reset will do

It won't be accurate, and you will have to correct the time daily! Use on of the designs that has a crystal reference. Watch type crystals are common and cheap.

When I started reading electronic magazines "Everyday Electronics" and "Practical Wireless" many years ago it used to be called VERO-BOARD

I understand this part below and gave my views on this. First of all, I aquired this analogue multimeter, but I'm doubting it's accuracy

The issue in question was about the accuracy of his analog meter and not whether he should learn how to use it. I learned to use a slide-rule long ago but prefer my calculator today.

Why bother with an analog meter? Digital meters are available for a few $'s that will have orders of magnitude better accuracy than an analog meter. Analog meters usually have fixed 1% or slightly better custom value resistors in their divider circuits which makes any calibration difficult. Even the cheapest digital multimeters use laser trimmed resistances and good voltage references today. I grew up with analog meters and have collected about a dozen of them. They are now collector items. I will never sell them, or use them for any serious measurements. Also analog meters will give different readings laying flat or standing up.

I have used this before in HV measuring applications http://news.thomasnet.com/fullstory/14235/612

I will say this in a Q & A session:- Are there questionable design practices involved? Yes, whether we like it or not. This is just not the way things are done by power electronic designers. This may be due to the fact that this is not the area of expertise of the creator of this design. Does this flame the designer? No, he may be brilliant in other areas. I can write software that works but I suck compared to some other people I know. Will it work? Maybe under low power conditions for short periods. Will it work reliably under the claimed power for extended periods? Probably not. Can it be improved? Like everthing, yes. Here the best way will be to contact the designer and point out any potential problems and work with him towards a solution. Engineers in general are humble people and should not feel offended by this action. Most will welcome any advice to improve a design.

I would not use bipolar power transistors in a push-pull configuration period! (many hidden problems to the inexperienced, especially at high current levels) I would not use 50/60 Hz inverter transformers. (too bulky) The 4047 would be last on my list of choices. (hate RC timing constants, prefer XTAL) It would we a different design all together. (HF switching) I would use opamps only for feedback, protection and regulation functions and not for drivers. I am saying this based on my experience of designing a commercial 5kW UPS for one of my previous employers. I designed some 400Hz inverters for aviation applications in the 70's using bipolars but using proper precautions like using clever base drive circuitry, baker clamps, device desaturation detection, transformer saturation detection etc. Today most things are much easier and safer using power mosfets and dedicated drivers.

As I stated before, I use data sheets as a guideline only. I have saved companies loads of money by designing and redesigning products to use lower grade components. The important part is to understand what parameters may be exceeded under which conditions and to design around the limitations. The one thing that separates the Technician from the Engineer is the ability to design and evaluate using advanced circuit analysis and mathematics which goes beyond just

One was called stip-board magic. Company went belly up years ago, but program may still be available on the web.