Ramp Generator: Oscillator

Background

A while back, I purchased a Topward 8110 Function Generator from eBay at a pretty good price. The unit had a dent in the front panel and was marked as needing repair.   I was able to remove and then flatten the front panel. No other physical damage was found. I did have to replace two driver transistors to get the unit working. But at the end of it all, I was pleased with the unit. Figure 1 shows the front panel.

Figure 1. Topward 8110 Function Generator

One of the reasons I wanted this unit, was that it had a VCF input (Voltage Controlled Frequency). Using a ramp signal, the FG (Function Generator) could be made to sweep a range of frequencies (1000:1 range). Using a scope, it is possible to view a filter response as the frequency is swept through its range.

Some FGs provide this sweep capability as a built-in feature. But to use the Topward 8110 in this way, I needed a ramp generator circuit. So why not build it myself?

Oscillator Design

The Topward FG accepts a range of 0 to +5V in the VCF input. So I decided that the entire circuit would run from a regulated power source of +5V.

The sweep (ramp) frequency was chosen to be under 100 Hz. It didn't need to be high and there was no reason to make it higher. The requirements so far, allowed the use of a NE555 timer and some low voltage rail-to-rail opamps.

The ramp could be generated using opamps but I settled on the 555 timer in order to keep this project simple. This would be the main generator for the whole unit. The one feature of the 555 timer that I liked is that the timing capacitor is charged and discharged within a strict range of 1/3 to 2/3 VCC. Using a +5V supply, this means that the capacitor would operate within a 1.67V to 3.33V range.

The next design problem is that normally a capacitor charges according to an exponential curve. What I needed was a linear ramp. So a constant current source for the charging circuit would be necessary. Figure 2 shows the NE555 circuit.

Figure 2. NE555 Ramp Generator

The LED1 componentwas chosen to have a forward voltage drop of about 1.2 V at about 10 mA. Red LEDs usually have some of the lowest VF values, so a red one was used. In this circuit however, the VR1 turns out to be 3.28V due to the lower current flow (VF=1.72V). It would have been best if VR1 was 2/3VCC + Vbe or higher, but I found that it worked well enough (ideally VR1 >= 3.93V). Voltage at the emitter of Q1 was measured to be 3.38V, with its base at 3.32V.

The PNP transistor Q1, combined with the LED1/R1 divider forms a simple constant current source with the caveat that some loss of linearity occurs at the top of VC1. The remainder of the 555 timer circuit is your standard astable timer configuration. The output on pin 3 will provide a square sync pulse, while the ramp signal of interest forms across C1.

Figure 3. NE555 timer signals C1, and output Pin 3.

From figure 3 you can see the square sync output on the scope channel 2, which varies between ground and about 4.57V. The signal output goes high until the capacitor starts to discharge. At the end of the discharge cycle, the sync output returns high again.

The VC1 is the sought after ramp signal. It rises after the discharge cycle completes. The ramp is very linear except perhaps at the end. Any non-linearity appears unnoticable here. On average, the ramp ranges in voltage from 1.65V to 3.32V before being discharged.

Next Objective

This circuit is not usable yet. We can't feed VC1 into the FG until we:

1. Buffer VC1
2. Level shift it for greater range

The charging circuit would be disrupted if we tried sending VC1 to the FG directly. It needs some kind of a buffer circuit. Further, we desire the full 0 to 5V sweep range, rather than the limited 1/3 to 2/3VCC. This will allow a greater frequency sweep.

In the next instalment, we'll design the level shift and correct the voltage range.

Thanks for reading.

Panasonic SV-3700 Repair

I have this used Panasonic SV-3700 digital audio tape unit, which I've had for some time now. This was a unit that I really liked owning but the problem was that I hadn't used it in probably over a year.

The service manual is available from here: http://elektrotanya.com/panasonic_sv-3700_sm.pdf/download.html

I recently needed a source of digital audio so when I went to use it, the old adage became real - "If you don't use it, you lose it!". I could eject and insert a new tape cassette but attempting to play or record only caused the unit to stop and refuse to respond to the play/record controls. The only way I could retry was to eject and insert a tape again. And of course, I tried several other tapes with the same result.

After some sputtering and muttering, I decided to clear the workbench and see if a fix could be had. Figure 1 shows the unit opened up, with the transport unscrewed and placed sideways over a folded piece of paper to avoid short circuits. This afforded greater visibility for troubleshooting. The transport is seen in the center behind the front panel.

Figure 1. Opened Panasonic SV-3700, with transport moved for better viewing and servicing.

I was of the opinion that the belts were probably ok. After all, the transport seemed to load, unload, and rewind ok. The problem only occurred when a play or record function was selected.

With the transport repositioned so that I could better view the action of the tape as it wrapped around the heads, I noticed that the head cylinder was never spinning. Could it really be this simple?

Figure 2.  View of the head cylinder from the end of the transport.

Reaching in carefully with the unit off, I touched the cylinder on the corner between side and top, to avoid putting any oil on the head. I noticed some resistance at first as I tried to make it turn. Then it seemed to break free but still had a noticeable amount of resistance. It was time for a power on test!

I powered the unit on and loaded a tape. I set it to record and then I saw a reluctant but spinning cylinder! Shortly after, the spinning overcame all resistance and seemed to continue with a proper smooth spinning action.

Rather than buttoning up the unit right away, I chose to allow it to continue recording for an hour or so (the length of the tape). I wanted to make sure that any built-in lubricants had a chance to reach places where they were meant to be. I also wanted any current "stickiness" to be eliminated by active wear.

And then there was more..

After it successfully recorded for an hour or so, I stopped the unit and requested a full rewind. The unit surprised me by doing a slow rewind and then stopping after about 10 seconds. And then it would retry about three more times. After the third stop, it simply gave up.

This didn't look hopeful but I unloaded the tape. Then I manually turned the spindle drives back and forth with my fingers to overcome some other potential "stickage". 

I reloaded the tape but was not at first successful. Then all of a sudden during a slow rewind attempt, the spindle sped up to full speed and has worked ever since. I naturally spun the tape back and forth a few more times to make sure.

Epilogue

I have since put the unit back together and it continues to work ok (though probably needs a head cleaning). 

So if you have mechanical devices, particularly tape devices, take them for a spin today. If you don't use them, you may lose them.

Thanks for reading!

Poor Man's Digital PGA - Part 2 of 2

No testing is complete until you try the final configuration. In the earlier post, I had simply grounded the extra resistor R7 to switch in the higher gain. Figure 1 below shows a capture of the signal at its base gain of 2.

In this capture, all extra AVR resistors R4, R6 and R7 are in the tri-state condition. It can be seen in Figure 1 that some noise exists on the output. Counting the bumps, there appears to be 20 per cycle amounting to about 100 kHz. The AVR device here is operating at about 1 MHz, using its built-in RC oscillator.

Figure 1. Noise on PGA Output

For use with the MSGEQ7, this should present no difficulty since the switched capacitance filters will eliminate this. This may present a difficulty when used in other audio work, however.

The good news is that this noise is reduced as the AVR resistors get switched in for higher gains.

You can watch an AVR switch between four gains here:

  https://youtu.be/iqPBaHtLFI0

Thanks for reading!

Poor Man's Digital PGA

Playing around with the MSGEQ7 chips it was readily apparent that the audio input level was critical to its graphic equalizer display. There have been many analog AGC (Automatic Gain Control) designs used over the years. Some designs using an opamp and a FET can be constructed but can be difficult to get right for students without an oscilloscope.

With an eye to reproducible circuits for readers, I was looking for a single-ended opamp design that was digitally controlled. Digital circuits tend to be easier to manage with the minimum of equipment. So I got to wondering if I could manage this with the use of the LM358 (or the LV rail-to-rail part MC33202).

I wondered if a Digital PGA (Programmable Gain Amplifier) could be constructed with a minimum of parts. Figure 1 shows a typical AC coupled non-inverting amplifier design. Because it is single ended, it needs the voltage divider consisting of R1 and R2. If VCC is 5 V, then the junction between them will be VCC/2 or 2.5 V.  C1 AC couples the signal into the non-inverting input of IC1. The feedback circuit is however a bit unusual because the gain for DC and AC signals differ.

Figure 1. AC Coupled Non-Inverting Amplifier

Those who know/remember their opamp theory, know that a non-inverting amplifier's gain is set by the ratio:

Av = (R5/R3) + 1

But in this circuit the capacitor C2 blocks DC current. The feedback current through R5 is so small, it behaves as if the output was wired directly to the inverting input. The net effect is that the Vout = V- = V+ for DC. The resistor R3 has no DC path to ground and as a result, can have no DC effect on the output.

Yet, the AC gain is however, still affected by R3. So the AC gain of the Figure 1 circuit is indeed:

Av = (500k/500k) + 1 = 2

It is well known that either R5 or R3 has to change to affect a change in this gain calculation. The good thing about this circuit is that the DC gain is not affected by resistance changes as long as R5 remains reasonable.

To change the AC gain, we could change out R5 using some FET switches but this proves inconvenient. Likewise R3 is inconvenient for switching resistors in and out. But if you stare at Figure 1 long enough, you have to start asking yourself "wouldn't it be nice if we could move R3 to ground?" Then you could easily add or subtract resistances to change gain. It turns out that we can!

Figure 2 shows the circuit modified so that R3 has one grounded leg and the capacitor C2 is moved above it. In this case R3 and C2 are simply in series and can be moved around.

Figure 2. AVR Controlled PGA

With R3 grounded, we can easily add resistances in parallel with it. R4 if placed in parallel with R3, is equivalent to:

R3 || R4 ~= 83k

This change alters the gain:

Av = (500k/83k)+1 ~= 7

So with the simple act of adding in R4 in parallel with R3, we've digitally changed the AC gain from 2 to 7. R4, R6 and R7 were chosen to be 10% tolerance resistances giving the following selection of gains:

Value	R3	Gain	dBV
0	500,000	2.0	6.0
1	83,333	7.0	16.9
2	42,962	12.6	22.0
3	30,051	17.6	24.9
4	21,073	24.7	27.9
5	17,405	29.7	29.5
6	14,549	35.4	31.0
7	12,701	40.4	32.1

With all resistors enabled by the AVR device, the maximum AC gain is about 32 dBV (40). With none of the added resistors connected in, the gain reduces to a minimum of 2 (6 dBV).

AVR Digital Control

The digital controls coming from the ATmega328P (Figure 2) must set the outputs of PD5, PB6 and PB7 along these lines:

1. To enable a resistor, the port must be configured as an output port and have the level set to zero (low).
2. To disable a resistor, the port must be configured as an input port and have the pull-up resistor disabled (thus achieving a tri-state condition).

Note that a tri-stated AVR input here is never left floating. It will assume the potential of the high side of R3, which will be ground effectively. R3 never switches out of the circuit.

The 'mega ports in Figure 2 were chosen to avoid some otherwise usable ports. You can of course change the ports to meet your own needs.

Function Generator Test

Figure 3 shows the scope traces of the 5 kHz function generator (channel 2) and the IC1A output in channel 1. Here it can be seen that the gain is 2.

Figure 3. R4, R6 and R7 disconnected (off), gain of 2

Figure 4 shows R4 grounded to enable it in parallel with R3. The gain is nearly 7 (422mV/64.3mV=6.56).

Figure 4. R4 || R3, gain near 7

Watch the gain change between four settings, driven by an ATmega328P:

  https://youtu.be/iqPBaHtLFI0

Thanks for reading!

MSGEQ7 7 Band Graphic Equalizer Chip

I had bought a couple of MSGEQ7 chips in 2014 from eBay with plans to do something with them. So recently I found myself a bit of idle time and mounted one on the breadboard to try it out. I used the circuit straight out of the MSI datasheet (Figure 1):

Figure 1. MSGEQ7 Circuit

I had a little trouble getting this to work correctly, so I scoped pin 8, knowing this is not going to be accurate but should indicate if the RC oscillator was running at all or not (Figure 2). The trace indicated about 102 kHz, which is probably higher with the 10x scope probe removed. 

Figure 2. Trace of pin 8 (Oscillator)

The problem was that I was getting an output (pin 3), like that of Figure 3. This was with no signal applied.

Figure 3. Incorrect outputs (pin 3)

After trying several things including gain and frequency adjustments of the function generator, I decided to try the other chip (I always buy two for this reason!) This chip swap yielded the type of results I was looking for (Figure 4):

Figure 4. MSGEQ7 finally works (near 60 Hz)

The tallest signal represents the first frequency bucket which represents about 63 Hz (from the documentation). This will vary somewhat depending upon the internal RC oscillator.  I used a AVR generated oscillator to strobe this thing and used the scope's sync to drive the reset. You can see two complete traces in the middle with partials at the ends. Unfortunately the Rigol internal hardware frequency counter didn't like the low signal levels. The software frequency measurement likewise didn't grok the low function generator signal levels either.

Figure 5 shows the result of the function generator at 160 Hz. I found that the signal generator (B&K Precision Model 3050) needed to be on the 0 dB output range at about 10%. The chip was fussy about the input level going from no output to all high when the signal went out of range.

Figure 5. 160 Hz

 The next bucket is 400 Hz, shown in figure 6.

Figure 6. 400 Hz

 Figure 7 shows the 1 kHz bucket activated.

Figure 7. 1 kHz

 Setting the function generator to 2.5 kHz for the next bucket (Figure 8).

Figure 8. 2.5 kHz

 Figure 9 shows 6.25 kHz.

Figure 9. 6.25 kHz

 Finally figure 10 shows the last of the seven buckets activated with 16 kHz.

Figure 10. 16 kHz

The idea behind the chip seemed cool. I had planned to drive it from an ATmega328P, hooked up to a Raspberry Pi. But having tested it out I quickly realized that this circuit is much too finicky for a "simple project". What it needs is a circuit ahead of the MSGEQ7 with AGC to keep the signal within acceptable limits. Otherwise the MSGEQ7 will simply disappoint.

The MSGEQ7 is a CMOS chip. I was aware of this and took my usual care handling it. But the first chip may have been static damaged on the breadboard. If you choose to breadboard it, I recommend that you make all the connections without the chip and plug it into the breadboard last. This is the first damaged CMOS chip I've had in quite a long time (it might even have been damaged as part of the shipping process).

You can read more about the MSGEQ7 chip from this PDF file:

  https://www.sparkfun.com/datasheets/Components/General/MSGEQ7.pdf

Thanks for reading.

No iMac? Build a iRasp3

I've wanted to try this for a long time with the Raspberry Pi 3. The Pi 3 has built-in WiFi, making it a natural for an iMac-like terminal unit. I've held off because of work on my new book, which is will be out early next year (shameless plug here: http://www.apress.com/us/book/9781484224052).

Now that the Pi 3 is free, I just had to repurpose an old Viewsonic VG2230wm monitor with a Pi on the back (Figure 1).

Figure 1. iRasp front view.

I received the monitor some time ago from a friend without a stand. I eventually junked another monitor which had a stand that could be adapted to the Viewsonic. You can see it crudely attached in Figure 2.

Figure 2. Note the adapted stand for the monitor. Raspberry Pi 3 is mounted on the back with plastic spacers, using wood screws in the plastic casing.

The Pi is mounted on top of four plastic spacers, made from a pen cut with a small pipe cutter. The screw holes were carefully drilled over an area known to have some breathing space underneath. The monitor internals are covered in a metal shield, so I wasn't too worried about hurting anything (I had also been inside before).

The Pi 3 also has four USB ports. So even though I need two for keyboard and mouse, I still have two available. If needed, there is also a wired ethernet port.

I made sure to orient the GPIO header strip towards the bottom. This will allow me to use a longer ribbon cable down to the Cobbler-T for breadboard work.

Someday I hope to replace keyboard and mouse with wireless peripherals, since the Pi 3 also sports Bluetooth. In the meantime, I'll be looking for a shorter HDMI and audio cable.

Thanks for reading.

ESR Project Build - Part 6 of 6

This build is not done until the unit is assembled in its cabinet. I thought this last step would be a slam dunk. But I was terribly mistaken!

The W2AEW design called for a 5 volt regulator. Since the parts used didn't require 5V specifically, I tested it using a 6.5V switching supply that I had handy. After all the wiring was done, I merrily built this 7805 circuit on a tiny proto PCB assuming all would be well. Of course, once it was all nicely soldered into place the unit didn't function!  I could not get the meter to deflect with the leads shorted.  Wah, wah, wha... epic fail!

I soon discovered that if I raised the voltage, things returned to normal. So I tested it on a variable supply and found that it returned to normal over 6V. Being too lazy to rework the PCB much, I simply put a pair of diodes in the ground circuit to raise the output to 6.2V (Figure 1). If I was less lazy, I would have run a filter cap over the diodes also, but found that I could get away without.

Figure 1. 7805 regulator circuit used (for 6.2V)

My BNC test connector cannot be grounded, so I found a piece of saved orange crate plywood, painted it black and drilled a hole for it. I eventually got the unit pretty much done without the external case but ran into another problem (more about this later).

Figure 2 shows the internals of the unit -- large enough for an electrical outlet for the wall wart which delivers about 10 VDC without a load. 10V incidentally, is too much voltage for low ESR readings. So a regulator was needed.

Figure 2. Internals of the ESR unit. The PCB hanging down has the 7805 regulator.

The problem was the Fine/Coarse switch that I added to the front panel. When that switch was on Fine, a 47 ohm resistor was placed in parallel with the 10 ohm, to reduce the resistance ever so slightly from 10 ohms to 10||47=8.25 ohms. This provides wider spaced low resistance readings on the meter.

The problem turned out to be that the old switch I had used had an on resistance of several ohms! This drove me batty until I finally took the meter and read the switch resistance. So even with the 47 ohm resistor switched in, it ended up having no effect. After rummaging through the junk box again and measuring the on resistance this time, the issue was resolved with a better switch.

Figure 3 shows the unit re-assembled in the Sencore transistor tester case.

Figure 3. Assembled ESR meter. Zero pot at left, BNC for test leads in middle, power switch at upper right and Coarse/Fine switch below the power switch. BNCs at bottom are NC.

I suppose the lessons of the day includes:

• Don't assume the last part of the build will be trivial
• Don't assume switches will all measure 0 ohms, especially old ones!
• Test with the intended supply voltage up front.

Thanks for reading.