27 MHz Amplification Query

This is the circuit i am working around with.......

Sir the transmitter is drawing some 0.25Amps of current @ 5Volts input! transistor pin readings for transmitter as read from DVM is shown below (on normal operation):-


I provided some 12Volts to the transmitter and 5 volts to the receiver. The transmitter drew around 0.3Amps from the supply. Also when i placed the transmitter too close to the base of the whip antenna it showed some signal reception at maximum gain position on the preamplifier on the receiver. When i push the potentiometer knob on drain of BF998 to the minimum or mid position no signal gets received......... i think my transmitter is again oscillating at wrong frequencies....... what do u say??

I removed the 27pF and 500nH capacitor and inductor in parallel from Q5 (2n3866) from the transmitter and in place of it placed a 5.6uH inductor. The range now stands at 2feet with the 2N2219 transistor getting warm......

Also i built a very simple diode probe on a breadboard as shown below:-

I placed the diode probe before the antenna on the transmitter that is after the low pass filter and observed a potential difference of around 1.2Volts..... so i think the peak voltage would be around 1.8 V considering the 1N4007 barrier voltage of around 0.6-0.7V
Please help!

Also Should i place a common-base arrangement of any NPN transistor before the BF998 stage on the receiver in a cascaded fashion b/c the input impedance of BF998 is et at around 1Mohms (if i am correct). With a common-base arrangement i will get a low impedance at the input of the receiver for better impedance matching b/w antenna and the receiver........ but before this, i believe the existing system needs a lot of trouble-shooting!
Another thing which came to my mind is that is it possible that due to poor impedance matching b/w the transmitter and the antenna and also receiver and its respective antenna efficient power transfer is not occurring??

P.S. I dont know what is the impedance of the inductor loaded antenna i have placed........

Using a 1N4007 diode for your detector will not work as it is way to slow for rectifying 27 MHz. Use something like a 1N914 or similar fast switching diode. If the transmit circuit is working and provide full drive to the final 2N3866 you should be able to get at least 1W of RF out of it.

okay i replaced the 1n4007 with 1n4148 diode in the diode probe (circuit shown in the previous post).....got the same readings!

It is soooo frustrating to be a long way away when you know you can diagnose this kind of problem almost immediately with it in front of you and the right test equipment is at hand!

The fact that you are seeing a voltage at the antenna means something is being transmitted and this is confirmed by something being received as well. My guess, and that's all it is, is you are transmitting and receiving on completely different frequencies, possibly both are wrong. In theory the crystal should hold the transmitted frequency stable but there are other factors such as stability in the amplifier chain that could be causing a completely different frequency to reach the antenna. Similarly, almost any receiver at an frequency will 'block' (overload) in the presence of a strong unwanted signal if it is strong enough and that may be the receive effect you are seeing.

I think the next step has to be to measure the real frequency. For that you need a frequency counter to check the transmitter and a signal generator to check the receiver. Is there any chance you can beg/borrow/steal such equipment for a short time to do some more tests? If not, is there a possibility of building your own frequency counter? They can be quite simple and there are many designs on the internet that can be built very cheaply.

For the record, even with a short antenna, if everything was working properly, I would expect a range of several Km although the exact distance depends on many other factors.

Brian.

It is soooo frustrating to be a long way away when you know you can diagnose this kind of problem almost immediately with it in front of you and the right test equipment is at hand!

The fact that you are seeing a voltage at the antenna means something is being transmitted and this is confirmed by something being received as well. My guess, and that's all it is, is you are transmitting and receiving on completely different frequencies, possibly both are wrong.

Click to expand...

Nicely said. The problem is designing circuits comprised of multiple tuned LC stages without any means to verify resonance frequencies and monitor RF signal levels frequency selective. For decades, radio amateurs only had a simple grid-dipper for this purpose, but at least they had this tool.

Okay i will try to intrude into the electronic department and will check the transmitter frequency on an oscilloscope....... :twisted:
This frequency counter interests me...... can i build one using my arduino uno for this high frequency? It would save some parts if i use the console screen for it! :/

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I found a very simple built for a frequency counter using arduino but it was stated that its max frequency at input is 8MHz. What sort of changes should i do to make it work for me? or give me an alternate design.......
**broken link removed**

Thanks!

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but then i thought that arduino runs at 16MHz so its not possible for it to measure freq. @ 27Mhz....... just a thought!

RMMK said:

I found a very simple built for a frequency counter using arduino but it was stated that its max frequency at input is 8MHz. What sort of changes should i do to make it work for me? or give me an alternate design.......

Click to expand...

Build a pre-scaler to divide by 10 (or perhaps a power of 2). This can be a counter IC, several flip-flops, etc.

Amplify the signal coming from your antenna. Shift its volt level so it is recognized by your pre-scaler.

i think a frequency counter will be a bit more of a drag....... since for the pre-scaler it would require another order of parts! if you may share a simple schematic composed of very common parts then that would be another case....... till then I would rather stick with the oscilloscope, which would require a verbal spat with the electronics lab incharge. He just cant digest to see mechanical guys around! :-(

Your detector , change the 10 MF for a 100PF the 1 M for a 10k and as said the diode should be a fast diode NOT a mains rectifier.
If you twiddle the tuning does the reading peak?, does the TX PA current dip? have you got properly decoupled supply lines to each stage in your TX, feed the oscillator and buffer, each with a 100 ohms in series and a 1NF to ground. The 2n3866 is a vhf transistor so could be oscillating on its own if there is not enough VHF decoupling hung around it. It might be cured by a 100 ohm resistor connected directly to its base in series with the input signal. Its called a "grid stopper"
Frank

I tried placing a 100ohms resistor in series b/w the capacitor and the base of 2N3866 but no reception was achieved! I must also inform that it seems that the 2feet range i get is when i feed around 8Volts from a power supply which donot have any filtering done and seems to have large ripples! But when i supply current through two separate batteries to transmitter and receiver; the pair fails to tune with each other no matter what! Only signal reception is done when i touch the base of the transmitter antenna with the receiver antenna! My hypothesis is that the oscillator was oscillating due to the rippled power supply and fails to do so with the battery! Truth be told i think the circuit was oscillating on the supply (if i am not wrong!)
Anyways i will try to check the output waveform and its frequency on an oscilloscope by tomorrow! Also when i supply current through that power supply to receiver; it shows reception of signal even when no power is supplied to the transmitter for signal transmission!
From now on i will try to tune the circuits by supplying current from batteries only!
What i have understood from everyone's responses is that either oscillator is not oscillating at all or its on a very different frequency!

I was cautious about suggesting the current dip method because it may still work when the frequency is wrong, in fact tuning the output network could be tuning the output frequency!

The simplest frequency counter I can find is this one:

https://www.circuitvalley.com/2011/07/60-mhz-frequency-meter-counter.html

There are other similar designs but that one seems OK. One I found has serious design flaws that could result in smoke as soon as it was powered up!

PIC devices give direct access from pin to internal timer so they can count as fast as the internal timer can manage regardless of the clock frequency. Typically they max out somewhere between 50MHz and 60MHZ but I have heard of them working to 80MHz in some cases.

If you use an oscilloscope to measure the frequency, remove the connection from the 555 output and connect the resistor to the supply line instead so it produces a steady output signal instead of being chopped up by the 555 square wave.

Brian.

i went to the various labs in the electronics dept. in my uni and couldnt found any oscilloscope except for the one which was faulty with maximum frequency more than 20MHz. I was told that the RADAR & communications lab does have such oscilloscopes but they didnt let me enter since a project/ research is under way there! :-(
I will build the above frequency counter as shown in the link and then will inform you about it.......
Thanks!

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But in place of BFR91, can i use 2N2222A in the above frequency counter??

Go ahead and try the 20 MHz scope. It might show 27 MHz, although at reduced amplitude.

I was once amazed to see a 3 MHz waveform on my old 1 MHz scope.
I thought about how this could happen.
I decided the spec is for the frequency range where we get flat response, and then we get reduced response above that.

BradtheRad said:

Go ahead and try the 20 MHz scope. It might show 27 MHz, although at reduced amplitude.

I was once amazed to see a 3 MHz waveform on my old 1 MHz scope.
I thought about how this could happen.
I decided the spec is for the frequency range where we get flat response, and then we get reduced response above that.

Click to expand...

Thats encouraging! :-o Yuppee! I will try it tomorrow and will upload the results as soon as possible!

It's worth remembering that the bandwidth of an oscilloscope is an indication of the maximum frequency it can reliably measure voltage, not the highest frequency it can display. To some extent the two go hand-in-hand though as there is little point in designing an oscilloscope that can only inaccurately represent it's input signal. Personally, I would use a 'scope with at least twice the bandwidth of the frequency you are trying to measure.

Please note that an oscilloscope is not the best instrument for measuring frequency as, at least on the type you are talking about, it requires a degree of guesswork as to how the waveform aligns with the screen graticule and of course it's exact timebase calibration. A spectrum analyzer is the best instrument with a frequency counter coming in as a close contender. If you can, still build a frequency counter because it is a very useful item of test equipment, very easy to make and can be very accurate without needing special parts.

Brian.

But sir i couldnt find this BFR91 transistor...... can i use 2N2222A or BC547??
This tool has caught my attention sir and I am going to build this frequency counter anyway! But the tutorial doesnt contain the source code. Only the hex file is there......... it would have been great had he shown the source code!

Looking at your transmitter schematics, I keep wondering if you don't have more stages than you need. The LC tank circuit is able to generate large voltage swings at substantial current.



This is just a simulation of course. It shows the load getting 40V peak-to-peak. You may or may not be able to get this from a real oscillator.

I experimented with your Falstad simulation in post #94. I deleted all but the initial oscillating portion. I believe I persuaded it to yield even greater voltage swings. (I forget what values I ended up with because I closed the webpage accidentally.)

The LC tank yields the smoothest sinewave for you to tap from. We cannot be sure the transistor collector will give you as high a voltage, nor as smooth a waveform.

The load is the antenna. I don't know how to represent a transmitting antenna in Falstad's simulator. So I'm not sure if you want high voltage and little current, or low voltage and high current. It will depend on the antenna's effective impedance. You may need to experiment with coil-to-capacitor ratios, to match the antenna impedance.

are you basically saying my PA is a bad design? :bang: For the impedance matching; i was using a pi-network........
sir the i tried to tune the power amplifier at 50ohm output impedance using pi-network which also serves a s low pass filter!
Sir i used this many cascaded stages b/c i was trying to achieve 1Watt Power output..... with this many stages will it effect tuning the circuit?? on post #94 i made a mistake in the load...... i made it an inductor! Post #101 contains the most recent circuit diagrams........ plz do verify them!
And plz confirm can i use BC547 or 2N2222A in place of BFR91 in the frequency counter??
Also i checked on the 20MHz oscilloscope......... these was some kind of signal shown on the screen but it was something i couldnt actually read for calculating frequency since the oscilloscope was a manual one! but the peak to peak value seemed large i would say! Forgot to actually read the voltage! :-x

Thanks!

For low frequencies like 27MHz you can use those alternative transisors. For higher frequencies you really need a transistor with higher FT than them though.

Your PA is fine, I think the suggestion of more stages than necessary is valid. For 1W output you really only need the oscillator and two stages of amplification. Nothing is fundamentally wrong with your design though, I think its just a case of aligning everything to the same frequency.

Brian.