While the old tank circuit + germanium diode works and has the advantage of simplicity its not the most accurate solution - I used to use an identical one in my amateur radio days (G8IJV) but I've not been active for over 35 years (marriage/kids/work/other hobbies/etc.). Then we used a couple of transistors in a Darlington arrangement as an amplifier but the calibration was always a bit vague - it changed every time you looked at it! More recently, though not for amateur radio purposes, I've used a SDR dongle and a 1/4 wave dipole; it gives a direct output in uV, dBm or whatever you want. On a laptop with a gps dongle it makes measuring & plotting field strength patterns relatively easy...

 

While the old tank circuit + germanium diode works and has the advantage of simplicity its not the most accurate solution - I used to use an identical one in my amateur radio days (G8IJV) but I've not been active for over 35 years (marriage/kids/work/other hobbies/etc.). Then we used a couple of transistors in a Darlington arrangement as an amplifier but the calibration was always a bit vague - it changed every time you looked at it! More recently, though not for amateur radio purposes, I've used a SDR dongle and a 1/4 wave dipole; it gives a direct output in uV, dBm or whatever you want. On a laptop with a gps dongle it makes measuring & plotting field strength patterns relatively easy...

Good evening @Irving

Nice to see you are a Ham !
It's funny indeed to read you as we both have had a long time without any activity, exactly for the same reasons !!!.

I was quite active on 2 m but now am very disappointed because there is nearly no traffic.
The regulations here in France have changed so I can now be active on 20 meters the DX band.
However as I have to re-buy new equipments, and meanwhile I use the web SDR from a dutch site. And again not a lot of traffic : just a few Russian, Italian ... except during the contest

Is Irving your first name or a pseudonym ?

If you are OK maybe we could have a quick chat on Skype

73
Philippe

 

I used a TLC555 (or LMC555) CMOS 555-timer circuit to generate a square wave.
R3 and R4 form a voltage divider to attenuate the signal to about 200mV.
You may substitute the 555-timer circuit with a function generator.

The op-amp under test is configured with a moderate voltage gain of -10 (negative because the signal is inverted).
The input is AC coupled via C2 while the non-inverting input is biased at 1/2 Vcc with R7 and R8.

Vcc is a single supply, anything from 5 to 15V.

When examining the output of the op-amp on an oscilloscope, we are looking for four things:

1) output should be centered about 1/2 Vcc
2) voltage gain of 10
3) phase inversion
4) rise and fall times on the transitions


View attachment 251223

The first test is with an LM358 op-amp which has a gain-bandwidth product of 1MHz.
View attachment 251224



The second test is with a TL072 op-amp which has a gain-bandwidth product of 5MHz.
Notice the difference in the rise and fall times compared with the LM358. The phase inversion becomes more obvious.

View attachment 251225

Hi @MrChips

First of all thank you for tour schematic. Sorry to get back late to you.

One question about your circuit :
Is there a specific reason why you connect the output of the NE555 to the inverting input of the LM358 ?

I have seen your inputs too on my other thread "LM 358 single voltage : How to reduce offset voltage"

Philippe

 

Hi @MrChips

First of all thank you for tour schematic. Sorry to get back late to you.

One question about your circuit :
Is there a specific reason why you connect the output of the NE555 to the inverting input of the LM358 ?

Philippe

I used the inverting input because I wanted to confirm that the output is inverted. If it is not inverted then the op-amp is not working properly.

 

I used the inverting input because I wanted to confirm that the output is inverted. If it is not inverted then the op-amp is not working properly.

OK ! Very clever !
Thank you again !

 

OP-amps have a LOT of characteristics and the blinking LED tester seems to be mostly a Dead/ Not Dead sort of tester. Offset, drift, and noise seem like they would all be a bit important. For a field strength meter at 144MHz, the amplifier is only going to be amplifying DC from the detector, which is reasonable. It might be that the detector us using a slightly forward biased diode to greatly improve low-range sensitivity, which is a handy trick. The ability to handle inputs near the lower supply voltage can also be important.
And for a FSM the stability is quite important as well.