Amplifying a Raspberry Pi

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
OldskoolPirate
no manz can test innit
no manz can test innit
Posts: 142
Joined: Thu Feb 18, 2016 9:32 pm

Amplifying a Raspberry Pi

Post by OldskoolPirate » Thu Aug 29, 2019 11:43 am

I’m guessing amplifying a raspberry pi configured to broadcast fm is going to be a sproggy mess, but has anyone tried it ?

A raspberry pi offers audio streaming, rds and fm oscillation all in one.

Albert H
proppa neck!
proppa neck!
Posts: 1401
Joined: Tue Apr 05, 2016 1:23 am

Re: Amplifying a Raspberry Pi

Post by Albert H » Thu Aug 29, 2019 12:59 pm

Please don't do this!!!!

The "FM" output from an R-Pi is actually a harmonic from one of the outputs. It's truly filthy! You can use the R-Pi as a streaming receiver and stereo coder (with a fast sound card add-on), and for bonus points, you can generate RDS too!

An FM exciter can be a fairly simple thing - especially if it's built for one frequency. There are plenty of good, working circuits around. Take a look at the old NRG circuit with the balanced oscillator. If you get it accurately balanced and loaded correctly (not like the "Veronica / Arreff" mess), you can get a really pure Band II source with a ½f output to feed to a cheap and simple CMOS PLL. A couple of well laid-out and filtered stages of amplification will get you to a really clean and stable Watt.... You can build one for <£20 if you shop around for the parts.

A friend of mine is using a variant of this circuit with an SAA1057 PLL with a 2MHz crystal and ½f input to the PLL. He has a pair of BF199s in the oscillator followed by a further pair of BF199s in parallel to drive a 15 Watt Mitsubishi FET to 12.5 Watts - just three stages to a useful power level. His PCB is double-sided (the top is just groundplane) and it's about 12cm X 8cm. The frequency required is burnt into a little 8-pin PIC, so changing frequency isn't trivial, and there are no adjustments on the board whatsoever - tuning is accomplished by bending coils a bit and by selection of a couple of the capacitors. I've looked at his board on a spectrum analyser, and there's nothing nasty - it's really clean. He bought a big batch of components and manufactured 200 of the boards. He's selling them in the 'States and into South America, and the first batch sold out in a couple of weeks! The only returns he's had are from people who needed a frequency change.....
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

OldskoolPirate
no manz can test innit
no manz can test innit
Posts: 142
Joined: Thu Feb 18, 2016 9:32 pm

Re: Amplifying a Raspberry Pi

Post by OldskoolPirate » Thu Aug 29, 2019 2:47 pm

Yeh I thought it would be lol. Was just wondering. Thanks

biggiedan
big in da game.. trust
big in da game.. trust
Posts: 91
Joined: Mon Feb 23, 2015 7:07 pm

Re: Amplifying a Raspberry Pi

Post by biggiedan » Fri Oct 11, 2019 12:57 am

OldskoolPirate wrote:
Thu Aug 29, 2019 11:43 am
I’m guessing amplifying a raspberry pi configured to broadcast fm is going to be a sproggy mess, but has anyone tried it ?

A raspberry pi offers audio streaming, rds and fm oscillation all in one.
Maybe this could be an alternative solution.

Gigahertz
no manz can test innit
no manz can test innit
Posts: 156
Joined: Fri May 08, 2015 6:42 pm

Re: Amplifying a Raspberry Pi

Post by Gigahertz » Wed Oct 30, 2019 7:44 pm

Yep as Albert has mentioned I wouldn’t amplify the raspberry Pi let alone use it on its own.

I’ve recently tried Pi_Fm_Rds file and the output of the Pi can be heard on other spot frequencies across the broadcast band!

I found with roughly 80cms of wire it would broadcast up to 500metres and I measured the output on a Lake electronics power meter “ how accurate it is???” about 25mW at 76MHz and about 10mW at 108MHz.

Rather put the Pi to better use....

Albert H
proppa neck!
proppa neck!
Posts: 1401
Joined: Tue Apr 05, 2016 1:23 am

Re: Amplifying a Raspberry Pi

Post by Albert H » Thu Oct 31, 2019 3:30 am

I've been using a Pi Zero W and an "Audio Injector Zero" card as a streaming server for an internet radio station.

I installed the cut-down version of Raspbian ("Lite") and added the drivers for the Audio Injector - a bit trickier than their website would suggest, but I got it working OK. I then installed Darkice and Darksnow (the configuration programme for Darkice) and configured the stream info as I wanted. I enabled SSH but didn't bother with VNC as I don't want to be running the desktop if I don't need to, and I've always found VNC to be pretty flaky!

The initial set-up was done with a little USB hub connected, a monitor plugged in, and a keyboard and mouse plugged in. Once I was satisfied that it was working OK, I disconnected all the peripheral bits, rebooted to the command line, and used Putty on my desktop machine to talk to the Pi. I set up a Chron job - to run on boot - to use the saved configuration file in Darkice, and another to set the "mixer" input setting to 56 - which seems to be about the right sensitivity for 775mV into 600Ω. I soldered terminating resistors on to the little RCA socket input board that came with the Audio Injector, and hard-wired the output of a little limiter board to the underside of it.

The Raspberry Pi Zero W with its connected Audio Injector card were housed in a plastic instrument case (from Farnell), along with the basic stereo limiter and level indicators. The whole lot is powered from a cheap "wall wart" linear 15V 1A power adaptor (from Rapid), and there is a 7805 providing the power for the little computer board, and a 7808 powering the bar graph indicators.

On boot, the R Pi begins streaming after about 30 seconds - the Zero W is very slow compared to the full-fat big brother R Pi. The audio quality is superb, and it sips current (compared to a complete desktop computer being wasted for the same job!). The whole thing cost about £35.

My next project (possibly this weekend) is to use the same hardware to build a stereo Outside Broadcast TX and RX pair. I'm going to try to include talkback if possible!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

neckertude
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 13
Joined: Mon Aug 12, 2019 4:26 pm

Re: Amplifying a Raspberry Pi

Post by neckertude » Thu Oct 31, 2019 8:19 am

Interesting stuff, Albert. I've been meddling with a Raspberry Pi 3b and Hifiberry card, its pretty darn good for the cost / performance. Would be very interested to read about your next Pi project :D

Post Reply