Aug 14, 2012

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Analyze This!
Dissecting and analyzing the 8640-Jr Signal Source!
A continuation in our Component Selection series

Many times when we see plans for a good project we think "Hey, I'd like to make this project but ..." 
we don't have the exact parts called for.  But if we dig around in the junk box for parts we do have
we can often use them just fine by altering the project specs or schematic accordingly.

This week's program focuses on ... Analyzing a circuit for component choice and usage.

Overview

Okay, time to roll up our collective sleeves and start applying the guidance shared over the last 2-3 episodes of CWTD.  We have a fascinating and extremely useful circuit that can be analyzed, diagnosed, enhanced, optimized, measured, and simplified ... all using the techniques we've been discussing during our recent "component selection" programs.  We think you'll love this exercise!  And better yet, you'll get the hang of looking at circuits the way designers do when it comes to component selection and making custom enhancements.

73, George N2APB  & Joe N2CX

Audio Recording ... (Listen to the MP3 podcast)

Discussion Notes:

<20:00:51> "Rick K3IND": I like the new CWTD page header!
<20:00:52> "Ray K2ULR": <http://dl.dropbox.com/u/43021514/CWTD/Aug%2014.html>
<20:01:42> "Armand WA1UQO": The new banner for the Teamspeak homepage looks great!
<20:09:35> "George - N2APB": Hope everyone has our whiteboard up on your screen, as we'll be talking directly to the schematics on the page. ... http://dl.dropbox.com/u/43021514/CWTD/Aug%2014.html
<20:14:15> "Joe N2CX": BTW the neat schematic we present here was drafted by Paul Harden NA5N
<20:31:20> "Joe N2CX": Oscillator runs ar 13.6 to 30 MHZ
<20:33:36> "Pete - WB2QLL": What does D1 do?
<20:33:53> "Lee KM4YY": What does D1 do?
<20:34:13> "Paul KD7KDO": How much current does the VFO draw?
<20:35:34> "Joe N2CX": And software costs $$$$$
<20:37:07> "Paul KD7KDO": So you get a pretty good sinewave?
<20:46:04> "Paul KD7KDO": How many turns is in this transformer?
<20:48:01> "George - N2APB": 12T = 12 Turns
<20:50:01> "Rick K3IND": what about R17?
<20:50:43> "George - N2APB": provides bias for Q4
<20:51:58> "Mike WA8BXN": what kind of feedback is there in Q4 with R17 connected to the collector rather than +5V?
<20:58:34> "George - N2APB": Haiku = Hi-Q
<20:58:44> "Paul KD7KDO": Shouldn't the input and output impedance of the filter be set at the same 50 Ohm impedance?
<20:59:26> "Joe N2CX": A tad of signal feedback but the main reason is that the bias gives you good gain to square up the input sine wave
<20:59:57> "Al K8AXW": Can the NP0 caps be replaced with silvered mica?
<21:00:14> "Joe N2CX": Normally the filter is 50 ohms at both ends. I presume that W7ZOI found that a good termination at one end is sufficient.
<21:00:44> "Joe N2CX": Silver mica is fine if you have them or lots of $$$
<21:01:40> "Pete - WB2QLL": That probably should be RXTX 6.3....
<21:08:36> "Mike WA8BXN": how was the number of turns picked for T2?
<21:09:47> "Joe N2CX": Enough turns to give high reactance of the transformer winding to preserve good low frequency performance.
<21:10:36> "Joe N2CX": Rule of thumb is that a transformer inductance needs to be at least 4-19 times the operating impedance level.
<21:10:49> "Joe N2CX": That's 4-10 times...
<21:12:18> "Paul KD7KDO": Can you comment on shielding needed for something like this ... orfor the attenuator if you put one at the output...
<21:14:05> "Joe N2CX": TIf you intedn to use an external attenuator the whole thing needs to be mounted in a good shielded box to eliminate leakage.
<21:15:22> "Peter SV0XAW": found this on youtube http://www.youtube.com/watch?v=abFqtb6u418
<21:16:46> "Mike WA8BXN": will any kits be ready for the in person meeting?
<21:16:49> "Joe N2CX": Thanks Peter I'll cheeck it out after the session.
<21:16:51> "Armand WA1UQO": Any update on the Paul Harden data book?
<21:17:42> "Joe N2CX": No news on the Harden book.
<21:17:59> "Joe N2CX": Don't know if there will be kits for the in-person meeting.
<21:20:35> "George - N2APB": Search for the "Wayback Machine" to look for a way to find information from websites that are no longer in existence.
<21:20:38> "Al K8AXW": Good job gentlemen!!
<21:21:49> "Al K8AXW": what is the url for the yahoo group?
<21:22:58> "Frank N3PUU": Great job guys! 73 and see you on Saturday..
<21:23:29> "Al K8AXW": TY George AR K
<21:23:43> "Joe N2CX": CWTD-subscribe@yahoogroups.com


SESSION NOTES

Do schematics sometimes look like this to you? ...

Well, it doesn't always have to be that way.  If you can break the schematic down into functional blocks and then consider the role of some key components, you'll get an idea for the way the project is intended to operate.  And THEN, you'll have a better handle on how you could go about modifying it to better suit your intended use.  Or perhaps instead of immediately needing to buy new components, you might have a better feel for finding something in your junk box that will do the job equally as well!

So let's get into it straight away and first consider a very useful and powerful circuit from W7ZOI called the "8540-Jr Oscillator". 


ANALYZE THIS!!!

Fig 1. Schematic for the signal source. (Note that this schematic was updated on March 29th to include
two resistors [30 and 43 Ohm] that were missing in the common base buffer, Q3. They are probably NOT necessary.)

 

 

Discussion highlights from the Podcast ...

         Designed by Wes Hayward W7ZOI

         Inspired by legendary HP8640 signal generator

         Circuit Functions

     Overall – Tunable reasonable quality test oscillator

     Tunes 3.4 – 30 MHz in three bands

         Main tuning and bandspread

     Good buffering and filtering for stability and signal purity

     Hartley oscillator with air-variable tuning capacitors

     Range 13.6-30 MHz sine wave output

     Divide by 2 and low pass filter for 6.8-15 MHz

     Divide by 4 and low pass filter for 3.4-7.5 MHz

     Voltage regulator aids frequency stability

     Buffer amps help maintain stability and low harmonics

         Voltage regulator

     Integrated circuit 7806 gives good regulation

     SEPARATE VOLTAGE REGULATOR FOR OSCILLATOR TO KEEP OUT DIGITAL NOISE FROM DIGITAL CIRCUITS

     6v REGULATOR VS 5V TO GET MOST PERFORMANCE FROM Q1 AND Q2

     Ceramic bypass caps in and out for filtering and eliminate instability

         Variable oscillator

     Hartley circuit for wide tuning range

     Air variable tuning caps for good stability

     365 PF TC1 (TUNING CAPACITOR 1) IS MAIN TUNING

     35 PF TC2 IS FINE TUNING

     JFET for minimal circuit loading

     2N4416 IS JUNCTION FET

     HAS HIGH GAIN AND LOW STRAY C UP TO VHF

     OTHER FETS – 2N3819, 2N5485, MPF102 WILL WORK BUT HAVE WIDER SPREAD OF CHARACTERISTICS SO MAY NEED SELECTION

     Diode on FET gate provides amplitude self-adjustment

     ALONG WITH C8 AND R4 RECTIFY SMALL AMOUNT OF OSCILLATOR SIGNAL AND APPLY NEGATIVE FEEDBACK AS OSCILLATOR OUTPUT TRIES TO CHANGE

     Iron core toroid for tuning inductor for good stability and low loss

     NPO caps in tuned circuit for stability and low loss

     C5, C6 C7 AND C8

     100 ohm resistor in drain circuit to prevent VHF instability

     SINCE JFET HAS GAIN UP TO VHF EVEN WIRING CAN ACT AS TUNED CIRCUIT.  10 OHM RESISTOR R5 “DE-Q'S” THE STRAY INDUCTANCE

         Two-stage follower/buffer amp isolates oscillator

     JFET Q2 follower for low oscillator circuit loading

     ANOTHER JFET WITH HIGH INPUT IMPEDANCE

     DRAIN RESISTOR R6 DE-Q'S STRAY INDUCTANCE TO PREVENT VHF OSCILLATION

     Bipolar transistor Q3 provides high RF gain 2N3904 USED BUT 2N4401, 2N2222 ALSO GOOD CHOICES

     CONNECTED IN COMMON GATE AMPLIFIER CONFIGURATION WHICH HAS VERY LOW OUTPUT TO INPUT STRAY C FOR GOOD ISOLATION

     DRAIN RESISTOR R12 PREVENTS VHF OSCILLATION

     XXXXXXX OUT drain and resistor resistors minimize instability

     independent RC filters in DC SUPPLY TO each stage for clean signals

     Output buffer uses wideband xfmr on ferrite toroid core

     bifilar winding for ease of construction

     2:1 IMPEDANCE RATION ASSURES THAT Q3 COLLECTOR SEES HIGH IMPEDANCE FOR BEST GAIN

         Highest 2:1 freq range feeds sine wave directly to output buffer. (13.6 TO  31 MHZ)

         Other ranges gotten from digital divider chain

     Additional RC filtering and 7805 regulator in digital DC power assure clean signals

     Second tuning range from squaring amp and divide by 2 flip flop

     SQUARING AMPLIFIER IS BIPOLAR 2N3904 BIASED AS LINEAR AMP BUT OVERDRIVEN BY SINE WAVE INPUT TO PRODUCE SQUARE WAVE OUTPUT

     DIGITAL DIVIDER 74HC74 TYPE DEVICE WORKS WELL UP TO 30 MHZ AND DRAWS LESS POWER THAN OTHER 74XX74 TTL TYPES

     FIRST DIVIDE BY 2 CIRCUIT PROVIDES SQUARE WAVE OUTPUT OF APPROX 6.8 TO 15 MHZ

     SQUARE WAVE OUTPUT IS CLEANED UP TO SINE WAVE BY LOW PASS FILTER

     SQUARE WAVE CONSISTS MAINLY OF FUNDAMENTAL FREQUENCY AND ODD HARMONICS (3, 5, 5 ETC.)

     Third tuning range additional divide by 2 flip flop (total of divide by 4) FOR 3.4 TO 7 MHZ TUNING RANGE

     Square wave output OF DIVIDERS cleaned up by low ripple steep cutoff LPF

         .07 Db CHEBYCHEV DESIGN

     Low ripple to keep output constant across frequency

     steep rolloff removes 3rd and higher harmonics of square wave

     Low pass filters use iron core toroid inductors and NPO capacitors for low loss

     MAY NEED TO CHECK INDUCTANCE AND CAPACITANCE OF EACH COMPONENT AND TWEAK INDUCTORS TO GET BEST AMPLITUDE STABILITY ACROSS EACH BAND AND MINIMAL  HARMONIC CONTENT

     Low pass use resistive input attenuation to preserve passband response

         output buffer uses resistive feedback for flat frequency response

     On both emitter and base BIAS RESISTORS ARE BYPASSED BY OTHER RESISTORS WITH DC BLOCKING CAPACITORS TO LESSEN NEGATIVE FEEDBACK THUS INCREASING GAIN  (OUR STATEMENTS IN THE PODCAST ABOUT FREQUENCY COMPENSATION OF RC WERE WRONG)

         Also bifilar wound broadband transformer on ferrite toroid

     2:1 TURNS RATIO (4:1 IMPEDANCE RATIO) KEEPS COLLECTOR CIRCUIT IMPEDANCE HIGH FOR BEST GAIN

     Once again RC decoupling in DC power feed

         Resistive attenuator on output for isolation and to minimize SWR mismatch.

         OVERALL OUTPUT LEVEL IS +7 DBM RMS OR 0.5v INTO 50 OHM LOAD

 

 

Description of the "8640-Jr Signal Source" from Wes Hayward, W7ZOI  (with permission from W7ZOI)

 

Another RF Signal Source: the "8640-Jr"

Wes Hayward, w7zoi, 26feb12, update 28/29Mar12, 9apr12.   (Click to download a PDF the article.)

 

Radio Frequency signal sources are like power supplies: we can always use one more of them. This signal source is based upon the Hewlett-Packard HP-8640B, which is one of the most popular generators available. The generator is no longer available new, having been replaced by more up to date synthesized instruments. But the 8640 is still available and popular on the surplus market. The HP-8640 uses a single variable oscillator operating at VHF. That source is then frequency divided to generate the various bands. The phase noise and stability get better as the division ratio increases. This divider chain basis is used in our design, although with significant simplification.

 

This signal source consists of a Hartley oscillator operating from 13.6 to 32 MHz. It is necessary to have a tuning range of at least one octave to really take full advantage of this divider topology. This range is available directly, or is divided by 2 or 4, providing output all the way down to 3.4 MHz. Each of the two divided ranges is filtered with a 5th order low pass filter. Only one such filter was used for each band. Each was designed for a cutoff frequency just above the top of the range. The passband ripple was varied to achieve convenient, off the shelf capacitor values in the filters. The harmonic suppression is reasonable, but is much worse than the instrument's namesake. The original HP-8640B has a stellar harmonic suppression of over 60 dB, which resulted from the use of 3 filters for each octave. The excellent suppression is atypically good for a signal generator. Our 8640-Jr signal source is shown in the schematic of Fig 1.

 

Note that we don't call this a signal generator. A signal generator is a well shielded instrument that can be used to measure the sensitivity (MDS) of a serious detectors of only modest sensitivity such as oscilloscopes. Not only is shielding good, but a signal generator is immune to interactions from other generators that might be attached. A modern signal generator has a well defined, low output impedance, usually 50 Ohms. Frequency stability is, of course, good. Our little offering does not fit all of these criterion. It is, however, reasonably stable. Moreover, the oscillator is well isolated from external influences, allowing it to be used with another similar unit for the evaluation of IMD of mixers or amplifier in the lab.

 

Photos of the rf source are shown below.

 

 

 

Examination of Fig 1 and of Fig 7.27 of EMRFD will show considerable similarity. In spite of this, this experiment provided a few interesting details which we will outline here.

 

The signal is extracted from the VFO with a source follower while it was pulled from the EMRFD signal source with a single turn link through the tank coil. The link is the preferred method, for the harmonic output is lower by over 20 dB. While the output amplifier contributes some of the harmonic distortion, the dominant contributor is the follower.

 

The secret of this design, although it is certainly not a secret, is the common base buffer. This was originally inserted in the EMRFD RF source when we tried to use the instrument to examine a crystal filter. The VFO would try to lock to the crystals when the output was reflected from the crystal filter and back through the output stage to the VFO. The extra buffer with it's excellent reverse isolation completely eliminated this difficulty. The rf source had not been usable for IMD measurements until the common base buffer was added.

 

The VFO in the '-Jr originally used a 2.2 pF gate capacitor. The particular JFET we were using and perhaps the gate diode combine to compromise the starting gain for the oscillator. The circuit would change output level when at the lower end of the tuning range. The changes were often in the form of sharp steps in output level as frequency was changed. Changing the gate capacitor from 2.2 to 4.3 pF completely eliminated this problem.

 

Note the method of extracting signals from the divider chips. In each case, a large capacitor provides DC blocking so the following resistors do not alter the DC conditions on the CMOS chip. A voltage divider then drops the level to that needed to drive the output amplifier. The 680 Ohm resistor provides a high Z load that the divider can drive while the 51 Ohm resistor provides the proper resistance to terminate the low pass filter. When first built, the 8640-Jr used a classic surplus variable capacitor from a WW-II BC434 Command Receiver. These feature a built in gear reduction drive (about 60:1) and a dial mechanism, all in a package that is surprisingly easy to mount and use. But there seemed to be problems with excess noise as the circuit was tuned. The capacitor was changed to another from the junk box, but the same problems were there. Eventually, they were traced to the inadequate starting gain mentioned above. But this had not been confirmed until the capacitor had been swapped out for the 365 pF AM broadcast band capacitor. The combination of two capacitors works well, although the band spread is extreme at the low end of the tuning range while almost inadequate at the high end.

 

It was recently (28Mar12) brought to my attention that an enterprising purveyor of radio kits has decided to assemble a sack of parts for this design and offer it to the amateur radio community. He says that he will use polyvaricon variable capacitors instead of the air variables that I used. This could lead to problems. See my Q measurement results on these parts at http://w7zoi.net/2faces/twofaces.html.

 

This design uses an output amplifier biased to about 20 mA emitter current. The similar stage in the EMRFD circuit, Fig. ,7.27 had a current of 35 mA. This means that the output stage of the EMRFD circuit could be driven harder. This means that either the output could be higher, or that a larger output pad could be employed. Both would be an advantage.

 

There is no adjustment of output level. Some sort of variable attenuator would be handy such as the pot adjusted circuit of Fig. 7.22. Alternatively, a PIN diode attenuator would do the job.

 

Spectral analysis of the output is interesting. The harmonic attenuation is poor as mentioned above and depends upon the band selected as well as the position within the band. The high band, even when neither digitally derived output is in use, still has a spur at half the output frequency, but it is at about -55 dBc. This is some of the digitally derived lower frequency leaking through. Experimentation and design refinement would probably improve this problem.

 

The schematic diagram is labeled with an output power of +7 dBm. This is approximate. The following data was measured, showing a slight variation from the nominal value:

       High band             +9.2 dBm at 13.6 MHz             +7.6 dBm at 32.6 MHz

       Mid band              +4.8 dBm at 16.3 MHz             +8.0 dBm at 6.8 MHz

       Low band             +6.8 dBm at 8.17 MHz             +8.2 dBm at 3.4 MHz

 

The low pass filters are purposefully designed for a cutoff that is close to the top of the respective bands. Hence, it is important to actually measure the inductance can

capacitance value, or otherwise confirm the proper operation of these filters. If the filters are suspect, they can by bypassed during construction and debugging.

 

 

 


REFERENCES

PARTS VENDORS QRP VENDORS
Digi-Key Midnight Design Solutions 
Mouser Electronics JUMA Kits 
Newark Electronics Kanga US 
Farnell Small Wonder Labs
Arrow Electronics Kits and Parts 
Future Electronics QRP Kits
Allied Electronics Dan's Small Parts
Jameco SDR-Kits
FAR Circuits QRPme
Radio Shack Flex-Radio Systems
ByteMark CW Touch Keyer
Texas Instruments Ye Olde Disk Shoppe
Philips Semiconductors EleCraft
Fair Radio Sales EMTECH
Barker & Williamson American Morse
International Crystal MFJ Enterprises
NTE Electronics S&S Engineering
Radio Adventures Oak Hills Research
Vector Electronics Whiterook Products Co.
Harbor Freight Tools Ten-Tec
Lashen Electronics Wilderness Radio
Surplus Sales of Nebraska Milestone Technologies
Radio Daze DIY Electronics
Antique Radio Supply Almost All Digital Electronics
Nationwide Radio & Eq. Vectronics
Parts Express LDG Electronics
Goldmine Electronics Ramsey Electronics
All Electronics Hy Power Antennas
B.G.Micro Amateur Electronic Supply

 


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