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Old 20th Oct 2018, 7:26 pm   #1
Bazz4CQJ
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Default 0.1-2000 MHz Wide Band Amplifier ~4

I'm posting this in "homebrew" not because I made it, but because of what it might be useful for incorporating in to other projects.

I stumbled across a Youtube video recently where some geezer said that he needed an amplifier to go in front of his $8 digital frequency counter and so he had bought an $8 wideband amplifier! A look on ebay found numerous suppliers of these units (see attached) and two of them arrived in the mail today, costing just over 4 each. This is the blurb;

0.1-2000MHz RF Wide Band Amplifier Features
-This product with wide frequency range, high gain, low noise figure
-This product can be applied to various rf receive front-end and increases communication distance
-Used for Short wave, FM radio, remote control receiver, such as cable TV signal amplifier need low noise rf signal amplification
Specifications
Size:25*52mm
Diameter of end: 5mm
Color:green + gold
Operating frequency:0.1-2000MHz
Amplifier gain:
F=0.1MHz, gain=32dB
F=500Mhz, gain=31dB
F=1000MHz, gain=29dB
F=1500Mhz, gain=25dB
F=2000MHz, gain=20dB
Maximum power output:+10dBm (10mW)
Power supply voltage: 6-12 VDC
System impedance:50Ω, SMA connectors.
Tips:
1.When working frequency is less than 500 MHz it get well gain flatness, can make it less than 1dB after careful adjustment. The lower frequency the higher gain consistency.
2.Amplifier working frequency of the lower limit is subject to input and output capacitor, the default value is 0.1 uF, working to 0.1 MHz. Increase the input and output capacitance appropriately, can extend the cut-off frequency, such as 10uF capacitance can work to 5KHz.
3.When the power supply voltage changes in 5-8 v, it can be used as a variable gain amplifier, gain increases with the increase of the power supply voltage, which suitable for radio frequency receive front-end circuit, using DA control power supply voltage, to control the gain of the amplifier, automatic gain control
4.When the power supply voltage in the 8-10 v, the low frequency end gain up to 30 db, at this time the amplifier has a low noise coefficient and good stability.
5.When the voltage is 12 v, reach maximum gain, the low frequency end gain of 32.5 dB.

Has anyone bought one of these previously? My first use is similarly for a DFC, for which it really needs a FET buffer in front of it, and a maybe a Schmidt trigger at the back of it? Might do well in front of a VHF tuner or DAB radio for someone who is struggling?

B
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Old 20th Oct 2018, 8:32 pm   #2
MrBungle
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Looks pretty simple. Basically an MMIC circuit from the datasheet. Sure it works fine. Incidentally you can get some really cheap MMICs from RS with similar capabilities for under 50p.

Annoying really as the big manufacturers will sell you a suspiciously similar looking “evaluation board” for 100 that does the same thing.
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Old 20th Oct 2018, 8:46 pm   #3
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

MMICs can be useful but they really need to have both their input and output terminated over their full design-bandwidth into a consistent impedance [usually 50 or 75 Ohms] or they can become unstable and sometimes oscillate.

(Remember that every amplifier ever buit has an aspiration to become an oscillator, by fair means or foul!)

Usual trick is to put a 3dB Pi-section resistive 'pad' on the output, using SMD resistors to minimise lead-inductance.
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Old 20th Oct 2018, 9:09 pm   #4
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

That seems to be the case for any high gain amplifiers I've found (hence the burn mark on my arm from a MOSFET exploding ). Attenuators cure so many ills.
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Old 20th Oct 2018, 10:19 pm   #5
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

If the MMIC is designed to be unconditionally stable then (in theory at least) you can connect any source or load impedance you like at any frequency an it will stay stable. But in reality, it's easy to throw away that unconditional stability by adopting poor PCB design especially if the MMIC has high gain at high frequency.

~30dB gain at 1GHz normally means the MMIC has two or more stages within the package and this calls for very careful PCB design. I would normally run a mile from a device like this at work because it has way too much gain for the stuff I normally do. But I'd expect to be using a very thin PCB dielectric with a device like this with plenty of fat grounding vias close to (preferably under) the device legs. The pictures on ebay suggest the PCB is a typical 1.5mm thick FR4 board so I'm not sure the microstrip dimensions are correct for 50R but this probably doesn't matter for most users. But this PCB material looks very thick and this will mean the inductance of the skinny via holes will become significant with respect to maintaining stability with various source and load combinations.

I'd also keep the top ground area under the device isolated from other top grounds. This might help keep the device stable when used into a poor match at >=UHF.

The little 180R SMD resistor might run quite hot if this device is biased from 12V. It's OK for casual use but it probably needs to use a bigger package or use more than one package to share the Pdiss. But at just 4 a pop it's hard to be too critical of a board like this.
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Old 20th Oct 2018, 10:58 pm   #6
Bazz4CQJ
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by G0HZU_JMR View Post
But at just 4 a pop it's hard to be too critical of a board like this.
Well the guy on Youtube seemed pleased and what's the cost of 2 SMA connectors?

Thanks for the advice re-stability. To my amateur's eye, the thought of putting a small metal screen across the board seemed like it might be worthwhile?

Buys like this are the sort of "stocking fillers" that can be lined up to play with during the long and dark nights of winter, and could either be a bit disappointing or very pleasing .

B
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Old 20th Oct 2018, 11:11 pm   #7
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Arrow Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by Bazz4CQJ View Post
Has anyone bought one of these previously? My first use is similarly for a DFC, for which it really needs a FET buffer in front of it.
No, I haven't bought one, but have seen them before and gave them some serious thought. But at that time, they cost a good deal more than 4. So that new price gives me reason for a further consideration.
For some applications, the 50Ω input Z will present a problem necessitating a Hi-Z to Low-Z buffer, which you mention. Depending on the required bandwidth, linearity, dynamic range, etc., such a buffer can be a considerable design challenge, which, by personal experience, I am well aware of.

Al.
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Old 21st Oct 2018, 1:31 am   #8
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by Skywave View Post
Depending on the required bandwidth, linearity, dynamic range, etc., such a buffer can be a considerable design challenge, which, by personal experience, I am well aware of.
Yes, I recall your efforts, and I think you did achieve some success. I think my interests (re the DFC) don't exceed 150MHz, and I was thinking of trying this front end, which has been published in one or two places. But then again, I recall that we looked at several RF VM circuits that had been published...

B
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Old 21st Oct 2018, 12:15 pm   #9
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Arrow Re: 0.1-2000 MHz Wide Band Amplifier ~4

Q1 has been drawn wrong: looks like someone had a MOSFET in mind. A 2N5485 is a junction FET, N-type.

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Old 21st Oct 2018, 12:36 pm   #10
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

It probably fits the footprint however.

On subject of counters, this looks interesting: http://lea.hamradio.si/~s53mv/counter/history.html
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Old 21st Oct 2018, 5:47 pm   #11
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

I bought a couple seven or so months ago, just to see if they were any use for SW listening, which they are but also amplify noise obviously. There were two patterns on there if I recall, one was as you and I bought, the other started 50?-4000mHz. I think some people were using them as part of an SDR set-up as well. I also saw one the other day that is a bit pricier and has a heatsink on the back, not sure what that's all about as it looks the same board.

I also saw Mikes video and use them from that as well now.

Andrew
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Old 21st Oct 2018, 9:21 pm   #12
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Thanks for the advice re-stability. To my amateur's eye, the thought of putting a small metal screen across the board seemed like it might be worthwhile?
For many applications it might be OK as it is but I think a MMIC with 30dB gain at 1GHz will require very careful PCB design to make it as stable as possible across a wide range of source and load impedances. I noticed a few versions of this amplifier on ebay and the PCB layout was different on these with only a few via holes around the MMIC. So these look a bigger risk with respect to stability if you wanted to use this amplifier as a general purpose bench/test amplifier.
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Old 22nd Oct 2018, 2:20 am   #13
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by G0HZU_JMR View Post
I noticed a few versions of this amplifier on ebay and the PCB layout was different on these with only a few via holes around the MMIC. So these look a bigger risk with respect to stability if you wanted to use this amplifier as a general purpose bench/test amplifier.
Out of my depth now; could you explain the significance on the number of holes around the MMIC? I think that if Tanuki's suggestion of adding a pad would be worthwhile, a 3dB loss wouldn't compromise my interests.

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Old 22nd Oct 2018, 7:40 am   #14
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Thanks for the heads up, I have just ordered one to play around with.

Peter
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Old 22nd Oct 2018, 9:24 am   #15
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by Bazz4CQJ View Post
Out of my depth now; could you explain the significance on the number of holes around the MMIC? I think that if Tanuki's suggestion of adding a pad would be worthwhile, a 3dB loss wouldn't compromise my interests.
Low inductance array of vias between top and bottom power and ground. Standard fast electronics practice.

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Old 22nd Oct 2018, 9:40 am   #16
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
Originally Posted by Bazz4CQJ View Post
Out of my depth now; could you explain the significance on the number of holes around the MMIC?
You don't just have to lay out a circuit board to be good at the frequency you intend using it at, you also have to use layout techniques good across the entire frequency range your active devices have gain at. The range of frequencies you don't want it to oscillate at is much wider than what you plan to use it at.

These MMIC amps are small and cheap and simple looking, but they have gain at frequencies where plated stitch holes in a printed circuit board used as connections to the ground plane look inductive enough to foster oscillation. So you need to parallel up a load of stitches and use ones of a suitable size to get good enough grounding to keep the little beggers stable.

You get the same trouble with microwave band transistors.

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Old 22nd Oct 2018, 3:13 pm   #17
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

OK, but do many holes indicate intentional good design, or a marginal design that had to be 'salvaged' by their inclusion?

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Old 22nd Oct 2018, 5:04 pm   #18
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

It depends on what is inside the MMIC and how much gain it has (up in the >GHz region) and what package it uses...

In this case I think there will be a two stage MMIC that gives 30dB gain at 1GHz and it uses a classic old SOT86 package. Normally, the SOT86 package is fine for most of the basic MMIC amps or 'modamps' in the MSA/MAR/ERA range. This is because there is usually a single stage amp with a pair of BJTs in a darlington config.

You still have to use several via holes close to the device ground legs with these classic MMICs but they are fairly forgiving unless you only fit a couple of vias on a thick PCB and space the vias a few mm away from the device legs. But it might take 0.5nH of via inductance to begin to see obvious problems.

However, if you have a two stage amp sharing the same SOT 86 SMD package and you have 30dB gain at 1GHz then the grounding requirements for the SOT86 package become much, MUCH more demanding. You could be looking at a net <0.05nH ground inductance requirement for the PCB vias in order to maintain good stability margins. It's a huge change in requirements for the PCB layout compared to a classic MSA/MAR/ERA MMIC from Avago or Minicircuits
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Old 22nd Oct 2018, 5:54 pm   #19
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Quote:
or a marginal design that had to be 'salvaged' by their inclusion?
I wouldn't start drilling more via holes unless it definitely shows instability in a chosen setup. Normally, a VNA is used to measure the amplifier to collect s parameter data and this can be used to predict if the overall PCB module is unconditionally stable at its SMA connectors.

However, one crude method to look for instability is to simply fit a few inches of low loss coax cable at the output SMA of the PCB and leave it unterminated/open at the other end of the cable. Then do the same at the input SMA with another short piece of cable. Then sniff the PCB with a pickup probe and look on a spectrum analyser on a wide span to see if the board is oscillating up in the GHz region. Then try fitting a short at the end of each cable in turn to see if it still remains stable. Then repeat with longer cables of a foot or so. This isn't as good as a formal VNA test but it is a quick and dirty indicator of any potential instability as it explores several combinations of reflection coefficient at the input and output of the board and by playing with cable lengths and shorts/opens at the end of each cable an unstable amplifier can often be revealed eventually...

If you don't have a spectrum analyser then look at the DC current draw from the PCB and see if it changes once you fit the unterminated cables. Often this current will change quite a bit once the board starts hooting. But this isn't a reliable test method.
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Old 22nd Oct 2018, 6:52 pm   #20
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Default Re: 0.1-2000 MHz Wide Band Amplifier ~4

Thanks for that info. If we were to assume that most of the hobbyists on this forum who might be interested in experimenting with one of these would probably be thinking of using it for something no higher than UHF TV, is the concern then that if it is oscillating at a frequency appreciably higher than that, it's going to be producing a pretty noisy background at the lower frequency? And as you say, without a 2GHz spectrum analyser , such oscillation could be hard to detect?

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