GMoretti@massey.ac.nz (Giovanni Moretti) (02/27/90)
I've build a spectrum analyser (0-100MHz) that has a 10.7MHz IF (from HAM-RADIO years ago) and it works quite well but I'm not happy with the log-amp/detector. The detector at 10.7MHz is cascade of amplifier stages each with the same gain (10dB) and a diode halfwave detector from each stage. The outputs of the detector are summed. Unfortunately it needs a lot preset pots to get it working correctly and its linearity is not as high as I'd like. Also I'd like to try making a direct conversion spectrum analyser - it would need to be DC coupled. The log detector would then run at a much lower frequency. I've tried using S-meter output of a CA3089 and its logarithmic but doesn't have the required dynamic range (> 60dB). How does one make real log amps that don't need lots of preset adjustment? Ideas ?? Giovanni -- ------------------------------------------------------------------------------- | GIOVANNI MORETTI, Consultant | EMail: G.Moretti@massey.ac.nz | |Computer Centre, Massey University | Ph 64 63 69099 x8398, FAX 64 63 505607 | | Palmerston North, New Zealand | QUITTERS NEVER WIN, WINNERS NEVER QUIT | -------------------------------------------------------------------------------
whit@milton.acs.washington.edu (John Whitmore) (02/27/90)
In article <596@massey.ac.nz> GMoretti@massey.ac.nz (Giovanni Moretti) writes: >I've build a spectrum analyser (0-100MHz) that has a 10.7MHz IF (from >HAM-RADIO years ago) and it works quite well but I'm not happy with the >log-amp/detector. Most log amps use the relationship that the base-emitter voltage of a transistor is proportional to the logarithm of the emitter current (sometimes approximated as the collector current). A good low-noise transistor will follow this "ideal diode" curve for about eight orders of magnitude in the collector current (power ratio 160 dB). Routine use of this effect is complicated by the temperature dependence of the equation, Ie= Is * exp( q* Vbe/( k * T) ) and a good logarithm amplifier either needs a second (matched) transistor at the same temperature to set the operating point, or a thermostat. The National Semiconductor Linear Applications databook has one thermostat-style solution, in AN-264. The thermostat they use employs three transistors, with one transistor doing the logarithm conversion, a second measuring the temperature, and a third acting as the heater. The thermostat approach does NOT require matched transistors. The second approach, with matched transistors is slightly less accurate (mainly because of mismatched emitter resistances at elevated currents), but works at lower total power dissipation. I think the National databook has one of these, also; AN-29, Figure 23. I would suggest using the cookbook circuits, since at the lowest current ranges these are not terribly easy to frequency-compensate (i.e. the op amps will likely oscillate), and these circuits have already been debugged. The second part of your problem is the precise rectification of the IF-frequency signal; all I can think of is a good hot-carrier diode, and keeping 60 dB of accuracy there is not going to be easy. Good luck. Your local National Semiconductor distributor (or the representatives in Australia: telephone (03) 729-6333) can supply the application notes I quoted above. I am known for my brilliance, John Whitmore by those who do not know me well.
johnw@watnext.waterloo.edu (John Wieczorek) (02/27/90)
A fellow from New Zeland wanted to know how to go about assembling a log amp. Probabley the easiest way is to get an Intersil ICL8048. I needs two resistors, a small 180pf cap and +/- 15 V and you're in business. It has 120 dB dynamic range for current input and 60 dB for voltage input. The voltage is just turned into current. The voltage precision is then limited by the offsets of its internal op-amps. You can build wider range circuits discretely with either chopper stabilized and /or low Ib op amps but the jump in cost and time from this chip in performance is about 5x money and 30x time. For you information: log amps and anti-log amps exploit the relationship that for a bipolar transistor Veb varies as the log of Ic. The chip packages it all very nicely for you. Analog Devices also has several entries in this field, but they are hybrids vs Intersil monolithics which translates to higher cost. I've used the chip, it's cute. John Wieczorek Disclaimer: The views and information contained herein are vicious lies, and are in fact taken from the drunken musing of my three, beer swilling, chain saw wielding but otherwise successfully house trained pet Blue Yukon mongooses.
michael@fe2o3.UUCP (Michael Katzmann) (02/28/90)
In article <596@massey.ac.nz> GMoretti@massey.ac.nz (Giovanni Moretti) writes: >I've build a spectrum analyser (0-100MHz) that has a 10.7MHz IF (from >HAM-RADIO years ago) and it works quite well but I'm not happy with the >log-amp/detector. > > >How does one make real log amps that don't need lots of preset adjustment? > >Ideas ?? > >Giovanni > Try Plessey. I can't find my data book but I'm sure they make some with at least 80dB dynamic range.
news@sun.Eng.Sun.COM (news) (03/02/90)
Reply-To: poynton@sun.com (Charles A. Poynton) Followup-To: Organization: Sun Microsystems, Inc., Mountain View, Calif., U.S.A. From: poynton@vector.Sun.COM (Charles A. Poynton) Path: vector!poynton Giovanni Moretti <GMoretti@massey.ac.nz> asks about a log amplifier over 60 dB at 10.7 MHz for a spectrum analyser. John Whitmore <whit@milton.acs.washington.edu> suggests the classic National Semi appnotes that exploit the logarithmic I/V relationship in a diode. John Wieczorek <johnw@watnext.waterloo.edu> suggests an ICL8048; this part uses the same approach. Both of these approaches are appropriate if the logarithm of a low frequency signal, perhaps up to 10 kHz, is required. However the problem in a spectrum analyser is to get such a wide dynamic range through the IF amplifiers unscathed; IF amplifiers are just not capable of 60 dB, so the log operation must be performed at IF frequencies, not after detection. As Michael Katzmann <michael@fe2o3.UUCP> suggests, Plessey is the place to look. They have a series of parts starting (SL521, 523, 1521, 1522, 1523) that are specifically designed as logarithmic IF amplifiers (for radar, in fact). In October 1980 these were documented in Radar and Radio Communications IC Handbook, but they're not in the March 1987 Radio Telecoms IC Handbook and I don't have the latest books. Plessey (NZ) Ltd., Auckland 8, Tel. 8364189. C. ----- Charles A. Poynton Sun Microsystems Inc. vox 415-336-7846 2550 Garcia Avenue, MS 21-10 fax 415-969-9131 Mountain View, CA 94043 <poynton@sun.com> U.S.A. "The most probable consequence of high tech jingoism is that it will keep the best of future technology out." -- The Economist, July 1, 1989, p. 19. -----