MRF1946 TEST CIRCUIT SCHEMATIC

[zulu53]

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Hi everyone!

It is a honor for me to share this forum with a lot of a knowable people. In the past, I have built few 3-4 watt FM exciters. Now, I would like to upgrade to 30W.

Motorola datasheet on MRF1946/1946A suggests a Broadband Test Circuit Schematic, (136-220MHz). According to datasheet, input power for 30W is 1-3W, depending on frequency.

Some years ago, I've got a pair of MRF1946A transistors but I never used them. Now, I would like to use them to build a 30W FM Broadband Amplifier. I was thinking that Motorola Datasheet would be a good starting point. And, I guess that " I just have to scale Motorola datasheet circuit schematic to FM Band." However, unfortunaterly, I don´t know how to go about it.

Therefore, I would appreciate if someone please could give me suggestions in this regard, because I don´t think that is enough to multiply every inductance and capacitance values by 1.81 (178MHz/98MHz) to have the circuit scaled to FM band.

Regards

[radium98]

like this but not broadband

[yellowbeard]

Have a look at this:
http://flaviobarbieri.blogspot.com/2015 ... ra-fm.html
It looks about right, but the low pass filter coils (LF1, LF2, LF3) should be offset at 45 degrees - more like the coils on this item:
https://dutchrfshop.nl/en/filters/13-1k ... ilter.html

[zulu53]

Thank you Radium98 and Yellowbeard. Apparently, you have suggested tuned circuits. I am trying to build a BROADBAND FM circuit because good trimmer capacitors are hard to find. I am quite sure that Broadband Test Circuit Schematic from Motorola, can be used for FM band. It only takes few changes on capacitors and inductance values. Can someone please suggested those changes? Thank you.

[zulu53]

Thank you Radium98 and Yellowbeard. Apparently, you have suggested tuned circuits. I am trying to build a BROADBAND FM circuit because good trimmer capacitors are hard to find. I am quite sure that Broadband Test Circuit Schematic from Motorola, can be used for FM band. It only takes few changes on capacitors and inductance values. Can someone please suggested those changes? Thank you.

[Albert H]

You're not going to find it easy to build a truly broadband Band II amplifier with a transistor like the '1946. It's not designed for that purpose. You will find that the gain will be very low, and if you try to get more power out of the device by putting in more power, you'll destroy it. You'll find that the wider the bandwidth of the amplifier, the lower the gain. That's why tuned PA circuits were suggested. They can be peaked for maximum gain at your choice of frequency.

[LeeCavanagh]

What about the 25w jira that Veronica/Nrg used to sell? That’s broadband, keeps your hands warm in winter too no need for a fan. Is there a schematic for him of that maybe....?

[radium98]

@LeeCavanagh.

[zulu53]

Dear Albert H,
It’s a great pleasure to have comments from a highly skilled and experienced RF expert.
My strong motivation to build Band II amplifier comes from the BROADBAND CIRCUIT SCHEMATIC, Motorola MRF1946 datasheet. Reading this document, I came to the conclusion that it was possible to build such amplifier, for different frequency band.
Unfortunately, in my previous posts I didn’t attach pages 3 and 4 of Datasheet. Please find them now. They show MRF1946 performance as a Broadband Amplifier for 136-220MHz band. Figures 2 and 3 are explicit. Furthermore, Fig. 8, ”Typical Performance in a Broadband Circuit” shows, 150-175MHz results with Pin 3W. That makes me still believe that MRF1946 could be used for a Broadband Band II FM amplifier.
Indeed, it is hard for me to understand how can a RF transistor used for a broadband ( 80Mhz +), high frequency set-up be totally unsuitable for lower bandwidth (20MHz) and lower frequency.
I would, therefore, appreciate if you could suggest changes in L1, L2, L4, L5 and C1-C4, C5, C6, C7,C11, suitable for Band II operation. During tests, Pin would be 4W, maximum. And, if for some reason the transistor burns, that will be one more lesson learned!
I thank you for your understanding.
Regards

[LeeCavanagh]

@LeeCavanagh.

Thanks radium

[zulu53]

Thank you LeeCavanaghfor the suggestion.
Thank you radium98 for the schematics.
Have you already built that Veronica 20W, 2 stages amplifier? Unfortunately there are missing capacitor values in the input matching circuit.

[LeeCavanagh]

Some pictures of the Veronica kits amp

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[Albert H]

That "Veronica" PA is a joke. The "filter" will do nothing useful (as the coils will couple one to the next), and it has a trimmer, showing that it's not actually "broadband" at all.

There's no Smith chart for the '1946 for Band II. The range of frequencies you want to cover - 20MHz wide in 108 MHz - is huge. The range of frequencies covered by the Band III application note is significantly smaller, the gain is only 8dB, and efficiency is below 60%.

Forget trying to make "broadband" amplifiers with devices like this. Bite the bullet and buy some suitable trimmers! The "Blue Spot" Philips parts will be fine for the output and the 65p yellow ones will be fine for the input. With careful alignment, you'll get up to 40 Watts out for 3 Watts of drive in an optimised circuit, and collector efficiency could be as high as 80%.

I could design you a "broadband" amplifier for Band II with this device, but it would be a waste of time and effort. You'd be lucky to get 15 Watts out of it and it would get hot.

[radium98]

when an rf expert like Albert H speask ,we have to shut our mouth .Thanks and thanks for inspirationphoto of veronica ,but once i saw a wide band for bly89c as i remmber or 6082 and another time for 1946A ,is tat true.

[nrgkits.nz]

I wouldn’t waste my time with these old devices, there’s much more modern ones like the MRFE6VP6300HR3, this is a 50v device and will easily give a broadband match at 300W out with only 1.5W input - the one I have on my bench runs with almost 80% efficiency.

[zulu53]

Well done LeeCavanagh. No-tune Veronica Kits amp looks very interesting. Apparently, it includes a trimmer resistor to avoid overdriving transistor input. Please, correct me if I am wrong.

I'm afraid I have been misunderstood. I didn't ask anyone to waste his time designing a broadband FM amplifier for me. I’ve just asked for suggestions on how to scale a well-established Broadband Circuit Schematic to FM Band II. Thank you

[Albert H]

I'll explain again. There's no point in designing a broadband circuit using an old, low gain transistor like that. Everyone tries to do this at some stage, and they all run into the same problem - lack of gain. The gain figure for your device is around 10.5dB at Band II, but that's in an optimised, tuned circuit. A broadband circuit will show around 5dB of gain, and low power efficiency. It's simply not worth the effort!

You really have two choices: -

You could build the PA for the frequency that it's going to operate on, optimise it using trimmers, then carefully remove and measure the trimmers, and replace them with a combination of ceramic capacitors to get the values you've measured.

You could buy a modern device that's designed for broadband use.

[zulu53]

I thank you for your very clear explanations. My circuit is intended to cover about 4Km in a rural, flat region, without hills or high buildings. I think 10-30W exciter connnected to a 30m high dipole would be enough. For the intended use, investing in 300W pallet, as suggested by nrgkitz.nz, maybe is not the best option.

Unfortunately, in this part of the world, only old RF silicon transistors are available. They are part of stock left in a company dealing with radio communications, mobile transmitters and base stations.

In the past, some people have build FM broadband amplifiers with some success using BLW60 transistors. They managed to get around 30W for 4W drive, which is not so bad for the type of used device. Therefore, I would appreciate if someone could share this circuit, because here BLW60 is still available in stock. Regards

[radium98]

zulu i think you must learn shearch with google

[Bton-FM]

I thank you for your very clear explanations. My circuit is intended to cover about 4Km in a rural, flat region, without hills or high buildings. I think 10-30W exciter connnected to a 30m high dipole would be enough. For the intended use, investing in 300W pallet, as suggested by nrgkitz.nz, maybe is not the best option.

Unfortunately, in this part of the world, only old RF silicon transistors are available. They are part of stock left in a company dealing with radio communications, mobile transmitters and base stations.

In the past, some people have build FM broadband amplifiers with some success using BLW60 transistors. They managed to get around 30W for 4W drive, which is not so bad for the type of used device. Therefore, I would appreciate if someone could share this circuit, because here BLW60 is still available in stock. Regards

It would probably be cheaper to get a 100w amplifier with an MRF101 than if would be to piss about with these ancient devices. Can’t you import some?