nrg pro3/4 audio responce

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nrg pro3/4 audio responce

Post by nrgkits.nz » Fri Feb 17, 2017 10:07 am

Just though I would post this as it could be useful for some people.

This is a simple mod i've come up with to flatten the audio response somewhat at the low end < 100hz stopping the pll from somewhat trying to correct the bass frequencies.

Testing was done on a pro3 original and a pro4. Stereotool was used for the test tones along with a Marian Trace Alpha sound card which is DC coupled and gives a perfect flat response on its output right down to DC, I verified this on a scope also.

A 1khz square wave was generated, the deviation on the nrg set to 75khz using a pira analyser. Next set the test tone to a 30hz square wave, deviation on the pira went upto a little above 90khz.

Now what I did next was add a 1000uF electrolytic to ground with a 100R series resistor to the loop filter, basically a lead lag configuration. It went in just before the 100k resistor R6 at the bottom of the board. Now the lock time has increased to almost a minute, however a 30hz square wave now deviates at around 80khz instead of more than 90khz resulting is less distortion and over mod because the PLL is not trying to correct the bass frequencies as much.

It could go even lower again by using a larger capacitor, but at the expense of more lock time required. Stereotool is supposed to be able to correct this by using its "tilt correction" feature, however its always good to have a transmitter which has a relatively flat response to begin with.

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Re: nrg pro3/4 audio responce

Post by Analyser » Fri Feb 17, 2017 5:06 pm

Sounds like an interesting experiment, but a couple of things... why did you use a square wave for the tests (these are full of harmonics)?
You also need to verify the response of the Pira analyser is accurate at those low audio frequencies before trusting the measurements.
You could try removing the PLL altogether and have a high value electrolytic in the audio path between your sound card and the mod input, this should give a relatively flat audio response you could use to check the Pira.

In any case this type of experiment is a great way to learn. With a lock time of about a minute you can see why engineers found way around the problem (the dual-speed PLL loop), having said that for a professional piece of equipment which is going to have its frequency changed very rarely, waiting 1 minute for it to achieve first lock isn't the end of the world!

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Re: nrg pro3/4 audio responce

Post by Maximus » Fri Feb 17, 2017 9:14 pm

I think the square wave generator is used for aligning the tilt setting in ST.


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Re: nrg pro3/4 audio responce

Post by nrgkits.nz » Sat Feb 18, 2017 11:42 pm

Thanks for the info, I'm aware of the harmonics using a square wave. Hans from stereotool uses a square wave in this video here: However he starts off with a 1khz sine then switches to a square wave down around 30hz. He also uses the Pira analyser.

Using a square wave at low frequencies you can see the pll's affect on it much more when doing a maximum hold on a spectrum analyzer which is what i've been doing also. A square wave forces the carrier to move and hold itself at the far most left and right points from certer frequency, hence why the pll tries to correct a square wave more than a sine.

I haven't verified the Pira frequency response, but I would say its more than likely within spec as Hans recommends it, plus it seems like a highly regarded piece of equipment from what i've read.

The problem with removing the pll is that there's slight drift until the thing stabilizes after a few minutes, the Pira also only support 100Khz steps, so its a bit of a mission to get the oscillator aligned without a pll, if its off frequency slightly it can affect the deviation readout. I didn't try a larger capacitor in the audio path though.

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Re: nrg pro3/4 audio responce

Post by Novaro » Wed Aug 08, 2018 12:50 pm

Any tip to make the same to the veronica pll 3 easytune?

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Re: nrg pro3/4 audio responce

Post by nrgkits.nz » Wed Aug 08, 2018 8:48 pm

Novaro wrote:
Wed Aug 08, 2018 12:50 pm
Any tip to make the same to the veronica pll 3 easytune?
Unfortunately you're wasting your time with a Veronica if it's one of scumbag Paul Hollings boards, he has no idea what he's doing and modified Stevens design in such a way that it will never perform as it should. Have a look at my site Gareth.net.nz, I've uploaded gerber files there for the NRG pro3, its a far better design.

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Re: RE: Re: nrg pro3/4 audio responce

Post by Electronically » Wed Aug 15, 2018 9:22 am

nrgkits.nz wrote:
Novaro wrote:
Wed Aug 08, 2018 12:50 pm
Any tip to make the same to the veronica pll 3 easytune?
Unfortunately you're wasting your time with a Veronica if it's one of scumbag Paul Hollings boards, he has no idea what he's doing and modified Stevens design in such a way that it will never perform as it should. Have a look at my site Gareth.net.nz, I've uploaded gerber files there for the NRG pro3, its a far better design.
I have to agree with nrgkitz.nz .Veronica was poorly made .I would go with what nrgkitz.nz said. nrg pro 3 is better

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