MSA0886 drive and max output level

[fmmpastouni]

Hello.I am having an fm oscillator that gives around 5 to 7 dbm.I want to drive a MSA0886 with it in order to get around 16dBm.The datasheet of MSA0886 says that output power at 1dB Gain compression is 12.5dBm.So I shouldnt exceed this output power level at MSA0886 in order not to produce signal distortion, harmonics, and potentially intermodulation products ,right?


Whats the gain of MSA0886 at FM frequencies and wthats the max output of MSA0886 in general?


Considering MSA0886 has a gain of 22dB at FM frequencies,is a resistive pi attenuator of 17dB ok to use between the oscillator and the MSA0886?

[reverend]

The MSA0866 datasheet has a gain of 32 dB at FM frequencies (at 36 mA standing current), not 22 dB. Note that if the standing current drops to 20 mA, the gain at FM frequencies drops to around 27 dB.

Assuming that you want +16 dBm output from a ~6 dBm input you will need an attenuator of approximately 22 dB (32-16+6).

However, with a 1 dB gain compression point of 12.5 dBm you might struggle to get a clean 16 dBm out and you will certainly need more drive, so something in the region of 20 dB ought to be OK and your 17 dB attenuator looks about right.

[Albert H]

Word of advice:

These little "pill" amplifiers are brilliant for buffers with a bit of gain, but when you overdrive them they get very dirty very quickly! You might be better off with a simple FET / Transistor "cascode" circuit - lots of input impedance, plenty of gain, and plenty of power handling - certainly much more than the monolithic device! You'll probably also find that it's a cheaper solution too.

[sinus trouble]

Take my comments with a pinch of salt on this subject? I have Zero experience with MMICs

I do find them intriguing nevertheless!

I imagine these devices would perform excellent in UHF front end applications or "Lab" Pre amplifiers etc...

It would be interesting to see what your project is that you are working on?

[Albert H]

One of my commercial designs uses an MMIC as the buffer after the "at frequency" oscillator. It provides gain, impedance transformation and isolation between the oscillator and the rest of the driver. It was more of an experiment than anything else, but we ended up making 80 driver boards using them! The object of the exercise was to minimise oscillator noise (particularly phase noise), and to that end I used a dual-gate FET for the VCO, buffered by the MMIC. The final results were (very slightly) better than my more conventional designs, but the additional hassle of an extra supply rail (the MMIC needed 6V) and the additional PCB space taken up by the supply filtering really wasn't a win. We went back to conventional (though Surface-Mounted) designs.

Bext used one for a while in one of their rigs, but it was notorious for spontaneous failures!

[jvok]

Mini-circuits do some very nice mmic amps that are super wide band and internally matched to 50r in/out. Some of them will do 100mW out with 20db gain on a 5v supply, so could probably replace 2 or 3 transistor stages in a 1-5w driver board. By the time you add up all the passives and pcb space you save, it would probably cost about the same for a lot less effort. You could have an RF deck thats as simple as oscillator -> mmic -> power fet, with all smd parts you could throw in a pll and mcu to control it and still have the whole thing fit in the palm of your hand.

I've used some of the mini-circuits parts before in my "real job" work and never had any reliability issues.

[fmmpastouni]

My project is building an fm transmitter.After having seached the internet i have two schematics one for the VCO and one for the amplifier.
As shown,MSA0886 is driven directly by the oscillator.The oscillator gives around +5 to +7 dbm.As is MSA0886 is not working in its linear region and produces harmonics ,but why the designer does so? It would be better to include an attenuator between the j310 output and the MSA0886 to limit its output at +12dbm max.What do you think?

PLL chip will be TSA5511.The PLL signal reference will be taken at the output of MSA0886.Do i need an L pad attenuator there or a pi type or just a series resistor and capacitor to pin 15 of TSA5511?

Those schematics are drawn by DRFS and PKBEE.

[jvok]

I think they're pushing it that hard to get enough drive for the RD01 fet. Bare in mind that for fm a bit of distortion doesn't matter. The final is probably running class c so will distort the signal anyway, but the low pass filter will clean it up.

[sinus trouble]

Nice circuit!

I think they went a bit crazy with the Varicaps? But thats not a big deal!

Q4 oscillator and Q5 buffer seems plenty adequate to drive the next stage? I do not see a reason why the MMIC is present in the circuit??

Its seems backwards to me?? Reducing gain to achieve gain??

Unless there is some extravagant impedance transformation going here? It dont make sense??

[Albert H]

They really know which circuits to steal, don't they? That 2-stage amplifier is remarkably close to my 10-watt driver from about 12 years ago. The coils are almost exactly the same, except that the use the VK200 (good luck with finding those at reasonable prices these days!).

[yellowbeard]

One of the PKBee 15 Watt transmitters used that exact VCO, but the following stage was a BFG591 before the RD15. They referred to the design as a "NiCom VCO". All the diodes are to ensure complete 88-108 MHz coverage without having to use a trimmer in the oscillator. I never saw one in the flesh, but I do have extensive files - the power control is done at the BFG591 stage and can be either set manually with a pot or controlled by a pulse width modulated signal from the PLL PIC chip so it can be set from the front panel. It looks a slick design, might be shite but it looks swanky. Here's a link - but be warned - there is no hex file!
https://www.fluxfm.nu/3-mtr-info-sales/ ... index.html

Another Dutch guy has a PCB drawn up for that VCO - and another similar design using a a different MMIC - scroll down this page and click "Suono"
http://mkmbroadcasting.blogspot.com/search/label/nicom

[fmmpastouni]

Nice circuit!

I think they went a bit crazy with the Varicaps? But thats not a big deal!

Q4 oscillator and Q5 buffer seems plenty adequate to drive the next stage? I do not see a reason why the MMIC is present in the circuit??

Its seems backwards to me?? Reducing gain to achieve gain??

Unless there is some extravagant impedance transformation going here? It dont make sense??

The use of so many varicaps maybe has to do with lower noise??
Maybe the choice of the specific mmic is wrong and one with less gain and higher 1dB compression point would be more suitable.Something like era-5,or era-6 and an attenuator between the VCO and the MMIC.

One of the PKBee 15 Watt transmitters used that exact VCO, but the following stage was a BFG591 before the RD15. They referred to the design as a "NiCom VCO". All the diodes are to ensure complete 88-108 MHz coverage without having to use a trimmer in the oscillator. I never saw one in the flesh, but I do have extensive files - the power control is done at the BFG591 stage and can be either set manually with a pot or controlled by a pulse width modulated signal from the PLL PIC chip so it can be set from the front panel. It looks a slick design, might be shite but it looks swanky. Here's a link - but be warned - there is no hex file!
https://www.fluxfm.nu/3-mtr-info-sales/ ... index.html

Another Dutch guy has a PCB drawn up for that VCO - and another similar design using a a different MMIC - scroll down this page and click "Suono"
http://mkmbroadcasting.blogspot.com/search/label/nicom

Yes thats the schematic but i prefer the schematic with RD01MUS1/RD15HVF1 than BFG591/RD15HVF1.I think the VCO is one of the best i have seen,and the other top VCO is with dual gate mosfet like BF900 as Albert H suggests.

They really know which circuits to steal, don't they? That 2-stage amplifier is remarkably close to my 10-watt driver from about 12 years ago. The coils are almost exactly the same, except that the use the VK200 (good luck with finding those at reasonable prices these days!).

I hope matching network part values are OK.

Word of advice:

These little "pill" amplifiers are brilliant for buffers with a bit of gain, but when you overdrive them they get very dirty very quickly! You might be better off with a simple FET / Transistor "cascode" circuit - lots of input impedance, plenty of gain, and plenty of power handling - certainly much more than the monolithic device! You'll probably also find that it's a cheaper solution too.

I had an issue of not being able to lock when i used the MSA0886 driven by -3dBm.The output was +17dBm.Standing current of MSA0886 was 26mA so a bit lower gain than the specified at 36mA.PLL reference for TSA5511 was taken at the output of MSA0886 via a 21dB attenuator.
When i used an attenuator at the input of the MSA0886 so as to achive a +10dBm at the output,it would lock happily at the whole band of FM.So when driven beyond the 1db compression point ,,the output got dirty and polluted the TSA5511.Thats what i guess. Bette stay 2-3 db below this 1db compression point to be safe...

About RD01MUS1....should input and output be terminated to 50ohm when measuring standing current or just leave them in the air...? If i leave it unterminated will it give self oscillations and increase the standing current?

[Albert H]

Don't run the RD01 unterminated. You probably won't kill it, but don't risk it. They have VERY high gain, and can be provoked to "hoot" very easily!

[RF-Head]

They really know which circuits to steal, don't they? That 2-stage amplifier is remarkably close to my 10-watt driver from about 12 years ago. The coils are almost exactly the same, except that the use the VK200 (good luck with finding those at reasonable prices these days!).

Please Albert show a picture of that then ?
I can tell you it is not a schematic from you!! because i know ho made the design and why somethings made like that in the schematic.

Your always telling i made this and i made that but you never showed any pictures.

[Albert H]

Hi. It's almost exactly the circuit I designed (based on the datasheets and the Smith Charts) for those devices. You will find those little driver amplifiers - doing about 10 Watts - inside and couple of products made by B**t. That's who they were designed for. They were in their product range for about 9 years.

My amplifier had one extra match coil and an extra capacitor between stages (it further flattened the response across the band). The amplifier was designed to provide a very clean, robust and stable source for a series of large valved amplifiers. In addition, there was an RF level-sensing circuit on the end of it that developed a DC voltage that was accurately proportional to the RF output, which was fed back to the post oscillator buffer stage (a dual-gate FET) to guarantee the drive power regardless of the frequency that was selected. The DC feedback voltage was also digitised and sent to the monitoring system as a 12-bit number. There was also a further version of the amplifier that covered the "Japanese" band (from 76 MHz), which had a slightly different filter configuration.

I can only assume that you took the same design approach, and came up with (roughly) the same values!