Local oscillator frequency too high

[Jolly 7]

Hello. I have been experimenting with this circuit. I do not have the specified ED1402 transistors. I got the local oscillator running, but am getting different minimum and maximum frequencies, which are too high for mediumwave reception. For example, using a 2N2222 transistor, I am getting 4.2 MHz instead of 2.1 MHz. I tried using a 2N3904, but the tuning range is incorrect. Is the originally specified transistor critical for this circuit ?
If not, what can be done to pull the LO frequency down and create an 1100 kHz tuning range to make MW reception possible ?

[Radio Wrangler]

Using a frequency counter?

It's possible that the LO contains a fair bit of 2nd harmonic ad that the waveform has a little kick in it. Counters can often count two edges per cycle and give you a doubled result.

It's one of the reasons the well-bitten engineer has a shufti with a scope first.

David

Adding to Davis comment, it is a mixer/oscillator and by design generates harmonics. I also doubt the transistor type matters at all as long as it oscillates (as it does).

[Jolly 7]

@ Radio Wrangler. Thanks. I don't have a scope. I use the frequency function of my multimeter and it reads the frequency generated by one of my 455 kHz test circuits accurately.

[frsimen]

The easy way to tell if you are running at the right sort of frequency is to use another radio. The local oscillator in the radio you are testing will be running at 455kHz higher than the frequency you are tuned to.

All you need to do is to take another MW radio and tune it to, say, 1200kHz or higher. Sweep the tuning of the radio you want to test from the low frequency end of MW upwards. You should hear the local oscillator as a hiss or whistle on the other radio at some point. If you do, you have proved that the local oscillator is running at the correct sort of frequency.

The waveform of your 455kHz test circuit is probably closer to a sine wave than the waveform of the mixer oscillator stage in your radio.

Paula

[Radio Wrangler]

Originally Posted by Jolly 7

@ Radio Wrangler. Thanks. I don't have a scope. I use the frequency function of my multimeter and it reads the frequency generated by one of my 455 kHz test circuits accurately.

Yes, it may be perfectly fine on reading frequency on some signals.

It is a counter. It counts how many times the signal passes through a threshold in a set amount of time. The problem is not with the counter, per se, but with an oddball waveform in the oscillator where the waveform has an extra wiggle to the cycle and the counter quite rightly sees two crossings in the right direction and it counts them.

Fancy frequency counters have adjustable thresholds, hysteresis and all sorts of functions you can fiddle with to try to avoid this problem, but you're adjusting blind without seeing the waveform.

Basic frequency counters just have an input , and that's what you get in multimeter ones too. OK with nice smooth sinewaves or square waves.

David

[trh01uk]

I agree with David that its your frequency counter - and a strange distorted waveform from your local oscillator that is giving you the 2nd harmonic (4.2MHz), rather than the fundamental (2.1MHz). If your counter has no adjustments for level, etc, as David describes, then the only thing you can try - is to reduce the coupling between the counter and the local oscillator.

I would do that by having a simple "probe", feeding into the counter, and then moving that probe about to see if the frequency suddenly reverts to 2.1MHz. A probe would typically be a coax lead, with a small loop on the end, and a suitable plug on the other to connect to your counter. The small loop can be a single turn of PVC covered wire, just connected between the coax centre and screen. Or a couple of turns. Play with it.

Richard

Sorry David, referred to as Davis, I must have over sampled. BAck on topic, a 'scope is one of the first instruments one should get. I got one of these https://www.ebay.co.uk/itm/FNIRSI-10...-/333616059342 for portable use, it works very well.

[Jolly 7]

Thanks everyone for all the replies.

While I am considering getting an oscilloscope, I ran some more experiments. I thought if swapping the MW antenna coil with another would make any difference and oddly enough it did. The maximum frequency is now roughly around 2.1MHz. I then set my Philips radio to 1215 KHz (now occupied by Absolute Radio) and was able to hear a loud whine with my oscillator circuit's tuning cap at its lowest tunable frequency. However, without turning my test circuit's tuning cap, I can also hear squeals/whistles on other parts of the dial of my Philips radio. They go away when I switch off my test circuit, proving that they are not intrinsic to the radio itself. If I turn my test tuning cap, the whistles also move to other parts of the band.

Here are some pictures of my test circuit and the radio I used to test it.