From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Adjustable current source
References: <3DFFF5AD.1D45@sneakemail.com> <3E00C854.7CC5@sneakemail.com> <3E0111D8.2D1D@sneakemail.com>
Date: Wed, 18 Dec 2002 19:40:05 -0800
NNTP-Posting-Date: Wed, 18 Dec 2002 19:40:08 EST
Organization: Bell Sympatico
Winfield Hill wrote:
> > Actually, the current may be closer to 200 mA. According to their spec,
> > the main coil sensitivity is 20 MHz/mA. Assuming they use a permanent
> > magnet to supply a bias field, the range from 2 to 6 GHz can be covered
> > with a 0 to 200 mA source.
> I saw that, but I had assumed for low-noise purposes you'd be glad to
> use a limited frequency-adjustment range.
> > The coil resistance is 20 ohms, so the voltage across the coil is 4V
> > at 6 GHz. Another 30 ohms could be added in series to adjust the
> > maximum voltage to 10V.
> Nope, that's not how I'd attempt to make a 200mA 10pA current source.
> Working from a 10pA noise-budget approach, let's assume 8pA from the
> resistor and the rest (6pA) from the amplifier/reference. We get 8pA
> from a R >= 4kT/i_n^2 = 250-ohm resistor, which means a current-source
> sense voltage of up to 0.2A * 250 = 50 volts. That's not so bad, but
> it will also dissipate 10 watts at 200mA full-scale, which seems just
> a bit strange for a quiet resistor!
> The voltage-noise budget for the reference & opamp = 6pA * 250 ohms =
> 1.5nV, which is tough, but not impossible. Create a "quiet" 50-volt
> reference (say 500nV), heavily filter it down to the 1nV level, and
> use a quiet 1.0nV or less voltage-noise opamp. We'll have to create
> a pair of tracking bootstrapped supplies for the opamp. Maybe a few
> other messy details. A veritable piece of cake. :-)
> - Win
Yes, everything is easy except for a few messy details:)
The FM coil is typically 400 KHz/mA with 400 KHz bandwidth. It is a piece
of cake once the main coil is done.
They sell a driver for the YIG filter version that might also work for
the oscillator. And I have their driver design on a CDROM around here
somewhere, but it's in Acrobat 4.0 so I need to find someone running W98
to read it.