I am trying to set up a relative signal strength meter for use with my xtal set experiments.

There are several designs on the web that call for 20ua meters. I checked inside to see if there was a shunt I could remove but there is not one. I think I am too far away to get a good reading. The readings I get are all under 1 ua, so the meter is just not valuable. Even reading mV would be fine but again the reading is just too low. I have tried this with several meters

What I am hoping to find is a simple circuit that can increase the voltage or current so it can be read. Accuracy is not important as it is only to see the relative impact of changes or adjustments.

Any thoughts? Google has not helped.

I have been using a op amp when replacing meters on tube testers. You can find the complete assy online for a couple of dollars.

DM

Hi DM,
Could you point me in a more specific direction such as the name of the assembly? I'm clueless if it does not have a filament and a plate.

Thanks
Carl

I used to use a 15 UA meter on my crystal sets , it worked fine for local and strong stations , but for DXing stations it was no help at all .

I'd start with an LM324 op amp because it's designed to be used for single supply. That way you don't need a pair of batteries. Set up as a minimalist DC amplifier, one op-amp and two resistors should do it. Google LM324 non-inverting amplifier. You're looking for this:



Gain is R2/R1

Thank you very much. That is exactly what I needed to know.
Carl

While you're at it, read the LM324 data sheet to make sure it doesn't need any special requirements such as grounding unused pins. I don't think you need to, but it would be simple to do and may be good practice.

I used something like this.
https://www.aliexpress.us/item/22518320 ... _shipto=US

DM

Thanks. I ordered one from eBay. See how it works!

The one I see on eBay has capacitors at input and output. If the one you receive has that configuration, you would need to short the capacitors for it to pass DC. A little microsurgery can be good for the mind.

You want a signal strength meter for assessing crystal sets right ? You are really talking about a sensitive RF probe, not a DC or audio spectrum amplifier.

One assumes you are working in the medium wave band from around 530 to 1650 kHz. The LM324 is a good OP amp for audio etc, but it is not much chop at all over 100kHz.

If you want to assess the signal, especially very weak signals, there is not much point in trying to do that by amplifying the signal from the detector, filtering that, and feeding that to a meter, even with OP amp amplification after the detector. The levels are too low for weak signals to activate the detector diode.

What you need to do is to amplify the RF signal that feeds the detector with an RF capable OP amp to get the signal, even for the weak stations, up to at least 500mV, before feeding that to a Schottky (or germamium) detector diode, before it is fed to the meter circuit, or you will not see any useful deflection on the meter with weak signals. An AD8056 is a suitable OP amp for the task.

When assessing the performance of crystal sets, the detector is an important part of it. So, if you only read the RF voltage prior to detection, you are overlooking a significant part of the receiver's performance. For this reason, the measurement is almost always done after the detector, at audio frequencies. Even a DC measurement will give a good indication of the receiver's overall performance.

The best tuning aid for a crystal radio is a tracking radio , a transistor radio next to the tank circuit , it as you tune the crystal set to a station and the tracking radio to the same station the tracking radio gets louder .

I just like having a meter on my crystal sets. Fun to look at even if not calibrated.

But that only works for strong signals. In the final analysis , if you want to improve the performance of the Xtal set, you need to look at weak signals, which may be below the detector's useful input range.

But then you might ask, if the signal is below the detector's input range, why bother with it at all, as you will not resolve it out of the detector anyway ?

The answer is that if you optimize what you have before the detector, then in the final analysis where a weak station could just be heard, or not, at some threshold, you will have a better chance of hearing it.

If you put the OP amp before the detector it becomes an active radio and no longer a passive crystal set . The whole point of building the best crystal set for DX is how far away that your set ,antenna , ground and headset .

If you put the OP amp before the detector it becomes an active radio and no longer a passive crystal set .
To optimize before the detector you need the Highest , Q tank coil , the best tuning cap n the best ant you can put up along with be best ground or counter poise have .
An active meter won't tell you what it is tuned to , it just might noise or the blead over from another station . Connecting the meter to the AVC voltage of a tracking radio would let one know what they are tuned to .

The relative signal strength meter and its op-amp are not considered part of the xtal set. The debate is whether the signal should be measured after or before the detector. Both have merit depending on what the OP wants to do. In either case, the meter circuit should have very high input impedance to preserve the Q factor of the set.

Yes that is why the signal would be coupled into the + input of an OP amp which has a very high input resistance and won't lower the Q of the tuned circuit, not to any significance at least.

Rather that coupling the output of that amplifier to a detector and meter though, I would simply look at it with an RF capable scope.

Over many years aligning radios, I stopped using the AVC or detector output as the indicator of the peak level. Instead I connect the scope very loosely coupled to the input of the detector, often just clip the scope probe to the plastic insulation if the wire leaving the final IF can, this creates about a 0.5 to 1pF coupling capacitor, the signal gets divided down by the 30pF of the x 10 scope probe, and look at the wave out of the IF. The timebase set so many waves are crowded together. It is much easier to see the peak signal level on the scope and adjust all of the tuning adjustments for the exact max peak level.

(On the topic of op amps, especially the differential amplifier, with the standard circuit where the 4 resistors are equal, not many people notice that the resistance looking into its two inputs separately are different. This is due to the feedback applied to the negative input. To get the input resistance to match up on both, requires that the series resistor and the resistor to ground on the + input are exactly 1/3 the value of the feedback resistor and series resistor on the negative input. It makes no difference of course if you are feeding it from a true differential, or isolated signal source. But if the drive is coming from two ground referenced signals, it puts a different load on each signal, unless this is corrected by altering the resistors on the + input).

Interesting discussion. As the OP, let me clarify what I am trying to do.

I want to be able to identify the strength of the tuned signal, for example, 1080 KHZ that is delivered to the audio stage, either headphones or AF amp. This will allow me to roughly quantify and evaluate the relative strength of different stations, band conditions, and the impact of coil or antenna changes.

I have seen several crystal set circuits that have a 20 ua dc meter just after the diode. My problem is that I am too far away from stations so I cannot get a meter reading that is at all useable. I get maybe 2 ua at best.

So given the device I have ordered.do I still need to short out the 2 input caps, and where do you see them on the board?
Thanks for all the discussion