A SIMPLE Voltage Variable Power Supply

I'm planning to include a 41db voltage-variable-attenutator in my HF Upconverter.  So I needed to build a tiny voltage-variable supply capable of 17vdc max down to 1.2vdc.  I discovered that using an LM117 was a nice way to accomplish this.  Since I needed something more than a voltage divider and it needed to have a fairly large range. 1.2vdc-17vdc.  

Input to this supply will be around 18-20vdc (I need to test what I need before I'm sure exactly)  I think there's about a 1.2vdc drop with the LM117 (something like that) so in my test feeding it with 13.8vdc output around 12.4vdc (from memory I didn't actually log it), but when turning the poteniometer all the way it went all the way down to 1.2vdc. 

The LM117 can accept as MAX input +40vdc [datasheet]

I simply followed the Datasheet for part values as a default.  Seems to work well.

Minus a few minutes to read the Datasheet, and solder the parts up this took about 30 minutes to build.  And seems to work JUST LIKE I HOPED.

I liked this so much I built a stand-alone Variable Power supply

Below is the start of a 13.8vdc INPUT and 1.2-38vdc @1.5amp Variable Power supply I've started building.  It's super simple to build this.  Takes just a few minutes with the right parts.

2 caps, 1 fixed resistor, and a 5k variable pot plus the LM117 and a tiny DC-DC BOOST converter capable of up to 40vdc output.  Input to the DC-DC boost in my case is a 13.8vdc supply that is common to a lot of things I already use in the shack.  Although it could be just about anything < 40vdc (approximately).

The LM117 max output is only about 1.5amp but this is more than enough for testing simple stuff in most cases (that I would be using something like this for).  Usually I'm trying to test a LNA and the mass of LNA's I have range from 5vdc to 28vdc all of them draw very few mA's.  So this is more than enough to use as a simple test bench variable supply for this.

This is a picture of the beginnings of my stand-alone supply.  I have a LOT of options I could incorporate with something like this, and probably will eventually.  I haven't added the DC-DC Boost supply to this yet, I had to order a second one since I already used the first one I bought.  These supplies only cost < $8USD on ebay.  The few other parts listed about are also < $8.00 for the most part.

I took the above pix after I was done testing one of my 5vdc LNA's

Space to the LEFT will contain the DC-DC Boost converter.  the LM117 is to the right with the small heatsink attached.  In this configuration I just connected it directly to the 13.8vdc regulated supply I meantioned earlier.  After I add the DC-DC Boost Converter it will connect to the converter, and the converter will then connect to the 13.8 vdc supply.

I could add a rotary switch with preset resistance with a bypass switch to the variable pot for convenience.  Then I could perhaps have presets of 2.5vdc, 3.3vdc, 5vdc, 6vdc, 9vdc, 10vdc, 12vdc, 15vdc, 20vdc, 24vdc, and 28vdc for convenience.  Just a matter of adding the switches, and some resistors.

I'll post more pix as this little project moves forward.

Touchstone RF Spectrum Analyzer Software

(ALSO SEE)

I wanted to have a quick and simple view of a large bandwidth that's being sent into my RTL Dongle.  Basically I wanted to SEE how altering the various low pass, and high pass, and band pass filters in my 100 Mhz Upconverter affected the output being sent to the RTL Dongle.  I came across this video showing how Touchtone's "RF Spectrum Analyzer Software" (I used the FREE version) could be used with one of these dongles to get a sense of how a filter is functioning.

(this is NOT MY Video, but demonstrates what I wanted to do basically)

Seems to work well enough!  I can now see what I wanted to be able visualize.

This was BEFORE I added a FM Broadcast band NOTCH filter AFTER the Upconverter

and just in front of the RTL Dongle.  NOTE the spectrum shape just above 105 Mhz is similar to the Band Pass filter shape I have at the input to My Upconverter.  LO Freq of 100 Mhz is at -65 dbm

This was AFTER I added a FM Broadcast band NOTCH filter AFTER the Upconverter and just in front of the RTL Dongle.  Notice the dramatic different in spurs around the 100.0 Mhz LO frequency and also the LO is down around -89dbm  Also the very different shape of the pass band around 109-115 Mhz.

While this example didn't show me what I'd hoped for, it DID show me reality of how this filter affected the results.  SO the lesson learned here is that this is a FREE and USEFUL Tool if you are attempting to visualize the effects of filtering.

I think what I'm actually looking for is a NOTCH filter that stop hard around 102 Mhz, however, this sort of thing can be quite difficult to find.  I'm basically trying to minimize the LO as much as possible or at least get it to be lower than the pass band from 109-115 Mhz. 

Anyway, I just wanted to share this with folks in case anyone else is trying to visualize the effects of a filter.  Of course, the thing to do REALLY is to inject a wide band noise source into the filter, and measure the output using software like this.  By the way, the "Ham It Up" upconverter has a noise source built in (minus a few simple addon parts to activate it.  It's a cheap way to get a decent noise source however.

RTL R820T on RF Analyzer App and Galaxy S5

RF Analyzer app Galaxy S5

Thoughts on the next bit of test equipment I'd like

Now that I'm just waiting for some small handheld antenna's in the VHF/UHF range to arrive.  I've been thinking how nice it would be to use an Android device that has RF Analyzer installed on it.  This is a VERY COOL app that'll run on Android and connect to RTL SDR, or HackRF One (I have both and have tested it with both, and it seems to work excellent).  

Here is a video of what I'm talking about.  

It's just a quick view of how well this works.  What I'm doing is using a small whip antenna that is good for 50, 144, 432 Mhz on a HackRF connected to my Android phone and running RF Analyzer @ 44.25 Mhz listening to AM Mode and the source of my nearby Power Line noise.

With the combination of being able to take video's, and photo's and being able to use the GPS and RF Analyzer all in one device (well two if you include the RTL or HackRF)...pairing all these tools up to be used as a portable RF noise sniffer, and recording device seems like the next logical step.

• RF analyzer paired up with a portable Arrow antenna and a HackRF to visualize the noise, as well as listen to it!  (YES the RF Analyzer app now can demod the audio, AM, FM, WBFM, SSB etc).
• Photos and video from the Android Camera (gps linked)
• On board GPS

On thing I've noticed about the HackRF is that holding it snug against the back of my Galaxy S3 increases the noise floor a fair amount.  So it's well know (I think) that the HackRF's case which is crap for RF shielding.  So I'll probably build a little Copper Clad PCB box to wrap around it or use some copper tape!  LOL

Anyway I'm envisioning all of this equipment mounted to the back of the VHF/UHF antenna, and while on site at a known noisy location all of this combined could help document a problem area.

The biggest issue I've had in the past with Power Noise tracking is the part where you need to document things each time.  With enough detail to be useful in debugging a problem.  Using all of this combined would make is as simple as waving your arms around with the gear doing all of the recording and minimal interaction required on the part of the user at the time.

This is also important from a safety standpoint.  For example you could actually be looking where you're about to plant your feet, or on coming cars, or big dogs, or downed power lines :-)  All kind of important!

The last time I'd used RF Analyzer it didn't include demodulation of the audio.  NOW it does and it CLEARLY works wonderfully!  Especially in AM mode which is helpful in power line noise hunting.

More to follow on this as soon as I've been able to video the operation of this...shouldn't be too long, I'm just waiting for my other android to charge up :-)