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 Post subject: Part 15 Transmitter using VCA?
PostPosted: Oct Thu 10, 2019 11:15 pm 
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Goldmine has this voltage controlled amplifier with a 6MHz bandwidth for $2.95.

https://www.goldmine-elec-products.com/ ... ber=G24178

CV input is logarithmic. Any guesses how it would function if connected to a crystal oscillator and feeding audio into the CV pins?


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 Post subject: Re: Part 15 Transmitter using VCA?
PostPosted: Oct Sat 12, 2019 7:36 pm 
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Not very well. The modulation envelope would be the logarithm of the audio signal - highly non-linear. This IC is meant as a solid-state equivalent of an audio taper volume pot.
John

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 Post subject: Re: Part 15 Transmitter using VCA?
PostPosted: Oct Mon 14, 2019 11:43 am 
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Joined: Dec Tue 29, 2015 8:23 am
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Location: Australia
There are linear (in V/V) VCAs/VGAs that can be used e.g. VCA824. Downside is they're about $10 a pop.

I tried one as a proof-of-concept once; AD9850 DDS followed by VCA824 into a 50ohm matching network & antenna, and it worked surprisingly well. IIRC, 100:1 (x10 max, x0.1min) gain range, better than 0.5dB gain linearity over that range, modulating frequency up into the 10's of MHz, and a good bit of drive capability too (something like 50mA+ & 6v p-p into a 50-ohm load).

With 100:1 you don't get 100% modulation, but you get pretty damned close…


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 Post subject: Re: Part 15 Transmitter using VCA?
PostPosted: Dec Wed 11, 2019 2:28 pm 
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Very interesting.


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 Post subject: Re: Part 15 Transmitter using VCA?
PostPosted: Dec Sun 15, 2019 2:21 am 
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Gee, I remember thinking the SAD1024A reverb chip was expensive at $11-12 at Radio Shack back in the early 1980s
$10 today isn't much.

Id love to see a AM transmitter circuit using the VCA824 :D

-Steve

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 Post subject: Re: Part 15 Transmitter using VCA?
PostPosted: Dec Tue 17, 2019 2:14 am 
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Location: Australia
azenithnut wrote:
Gee, I remember thinking the SAD1024A reverb chip was expensive at $11-12 at Radio Shack back in the early 1980s
$10 today isn't much.

True, it's not much - but it's still ~10x the cost of a MC1496 or discrete transistor modulator :lol:

azenithnut wrote:
Id love to see a AM transmitter circuit using the VCA824 :D

Haven't got any in stock, or I'd rig one up as before & measure/show the result. I need some VCA821's or similar for an upcoming project, so I might throw a couple of VCA824's into a quick post-Christmas order & do just that.


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