Compact Si5351 based SDR

This is a revised version of my FV-1 based SDR. I replaced the CS2100 clk generator with the Si5351 clk generator. The Si5351 has some advantages over the CS2100, namely you can generate quadrature clks directly. This simplifies the hardware design and improves the quadrature accuracy. The sideband rejection in LSB/USB modes is impressive..somewhere around 60 db as best I can measure. The DSP processing is accomplished by the use of a FV-1 audio processor. The device makes the base band signal processing a snap. It requires some code to be loaded on a EEprom but the circuitry is simple and allows for up to 8 selectable programs. I created three: AM/USB/LSB . The FV-1 provides for three analog POT inputs to control any parameters you choose. Gain, variable filter bandwidth and depth, AGC are some examples of adjustable parameters if you desire. I kept it simple and created fixed band pass filters to taste. I did use one of the controls for AF gain. The design has no tuned circuits or band pass filters but they could easily be added.  It works just fine without them. Occasionally, I come across a ghost signal from harmonic mixing, when tuning, but not enough to matter. The design uses an OLED display and a rotary encoder for tuning. The frequency coverage is from 2.7 Mhz to 25Mhz. The bottom limit is created by the inability of the Si5351 to support quadrature below this frequency. Although I have improved my DSP programs for the FV-1 and have developed new display drivers and the new code for the Si5351, useful detail about using the Fv-1 can be found in my original design from a few years ago: https://circuitsalad.com/2015/06/19/comming-soon-stand-alone-software-defined-radio-baseband-demodulator-no-computer-required/

Schematic:

The design uses a LTC6252 low noise op amp as an RF input with gain. It provides a constant and reliable resistive Rf termination for the sampling detector.  This allows for random antennas to be used without adversely affecting the input termination to the detector. All the code to operate the main processor(display/clk generator/tuning, band select and receive mode) was written in MikroC which is a C compiler for PIC and AVR processors. The generation of quadrature signals out of the Si5351 is not difficult to implement once you know how but..figuring that out took me a couple weeks of experimentation! You can connect switches, the encoder, volume pot and display directly to the main board for operation but I created a secondary board to mount the display and encoders. Instead of an analog pot and selection momentary switches, I used another microcontroller and two encoders(with one built in momentary push switch each) to create all of the switching signals, gain control, etc. This allowed me to have just two controls for all features.  The controls include: tuning, audio gain, mode, and tuning step. Tuning resolution is from 1Hz to 100KHz . For fun, I made the output of the FV-1 differential into the audio amp. This is not necessary.

Here is a link to all the files used to build this radio in a zip file(updated 1/18/20):

https://www.adrive.com/public/Fq3pNr/Si5351%20SDR%20Data.zip

The schematic and PCB was done with express pcb freeware. The C compiler used was MikroC, and FV-1 assemble was built in SpinAsm which is free and available from Spin Semiconductor(who makes the Fv-1). The gerber files provided were created for OSH park. I had my boards etched by them.  If anyone is interested in building this radio or leveraging elements of the design. I can answer questions.

Misc Notes: I use a 16650 3.7v lithium  rechargeable battery to power the radio. The current draw is about 100 mA with audio.  The radio works even when the regulators drop out so it will work at 3 v.

The enclosure is a machined aluminum 1590A style hammond box which you can buy on Ebay from alpinetech. They are $14.00 which is pricey but they are not cast. The quality is much nicer and you can anodize them.  It’s a different topic but home anodizing of aluminum is easy…and I do it with all my enclosures now. In this example, I anodized twice to create the base blue color and then the labeling as well. It looks really clean with this method. The nice thing about anodizing is if you make a mistake, it’s really easy to go back and redo the process.

Designers will note that the resistive terminations on the input RF OP amp contributes to the noise figure of the radio. As a practical matter. a negative impact on performance is not noticeable because of  atmospheric noise in the shortwave bands. For the best performance…no front end circuitry or a different front end input amplifier should be considered. Note that the op amp serves to bias the analog switches to half supply; so this bias must be provided to the sampling detector if the input termination is modified.  R10 set the impedance of the sampling detector, conversion gain, and low pass roll off. The schematic shows a value 0f 210 ohms…I think I am using 100 ohms actually now…which works well.

If you want quadrature out of the Si5351 below 3MHz you can create two outputs with 0 deg offset with one output at F and the other at 2F. You can then drive an analog mux with those signals and generate quadrature sampling for low frequency applications. Just note the output sequence of the samples change so you have to flip two outputs of the detector.

Top view of the circuit board:

sdr_top

Bottom View Showing FV-1 circuitry

sdr_bot

 

Display board

sdr_interface

Completed Radio

IMG_20190924_165851259

 

Demo Videos:

 

A very efficient half size 40 meter Vertical

I had an old broken 20 foot fishing pole that only had 16 feet of length so I decided to see if I could make an effective half size 40 meter antenna with it. Here is what I did. I used a capacitive hat to increase the radiation resistance of the antenna considerably. There are 8- 15 foot radials suspended a little less than a foot above ground. Finally, I made a small loading coil to tune the antenna up to the center of the band (7.100 MHz) for CW.  I first tested  a full size vertical to optimize the ground plane. By suspending the radials by only a few inches above ground, significant improvement was achieved . With 8 half size radials 10 inches above ground (compared to 4 on the ground), the measured loss went from about 20 ohms to  4 ohms. My loading coil has a measured Q at 7Mhz of about 300…with about 1 ohm of loss. The form was printed on my 3D printer out of HIPS..which is a low loss RF material. So my total system loss was about 5 ohms. With the HAT top load, the 17 foot antenna had a total impedance of 20 ohms.  This means I had a total system loss of only 1.25 db…which is not bad. the full size antenna had a loss of about .5 db … so really the difference is negligible. Finally I matched the whole antenna to 50 ohms with a little L network connected at the base. I like to use crimp style bullet connectors for all wire  connections because they provide quick disconnect and you can field repair(crimp) without need to solder anything(nice for portable setups).

I use it for QRP work on CW at about 1.1 watts. The antenna is a solid performer and it is very portable and easy to breakdown/setup. I can hear my signal on the many of the web based SDRs around the country. and have made numerous casual QSOs with it. Six or Seven radials work well also..you start seeing some more  loss when you go to four radials but it is still usable even then.

tophat

View of Top Load( 2 -8 inch strips of thin aluminum)

ant with loading coil

View of loading coil and base assembly

loading coil

The Loading coil

unmatched ant

Unmatched Antenna Z at Resonance

lmatchedant

The Final Matched Antenna Z

SmartPhone DI with Phantom Power

I frequently use my phone to record video for my blog and for my music projects…but I have been frequently frustrated by the limitations of the internal microphone embedded in the phone. So I created this circuit to provide a means to use a high quality Dynamic or condenser studio microphone with my phone. The circuit operates from a rechargeable  9 volt battery and plugs into the standard 1/8 inch four ring audio connector. It provides phantom power at 28 volts…which works fine …you typically don’t need 48 volts It utilizes a SSM2019 balanced microphone preamp IC, operating from a dual polarity charge pump power circuit at +-5V. At the output you need to provide a 2.2k resistive signature such that the pone can detect the microphone connection. The circuit also employs my simple momentary latching switch circuit which I often use and is described here at circuitsalad. Everything about the circuit is straightforward but the phantom power over voltage protection circuitry merits discussion. Basically, the isolation capacitors C5 and C6 are slowly charged up by the phantom supply but depending on what is plugged and unplugged can be be discharged very rapidly into the preamp input circuitry. This will be destructive and requires circuitry to shunt this energy to ground and dissipate it. This is accomplished by means of TVS1 and TVS2…which are prepackaged back to back zener diode. However the issue arises that simply using large zeners or transorbs is not a good idea because they have large reverse bias junction capacitance…which creates distortion as it modulates. To prevent this, I chose a very tiny 9 volt transorb. The one I chose (DF2B6.8ACT.L3F) has only a few pF of capacitance and is very small(402) surface mount package. The device works great but can only sink 1 amp peak. This further requires series resistors R11 and R16 to help limit the surge current to less than 1 amp at 28 volt. R1, R2, R4, and R5 must all be precisely matched in order to maintain circuit balance. R4 and R5 are required to provide a absolute ground reference such that the output does not float to some common mode DC value above 0 volts. The gain of the preamp is -6dB -20dB and is adjustable by means of R10. The reason for the negative gain is the use of an attenuation network that also provides the 2.2K resistive signature for microphone detection. The attenuation is required because of the inherent gain of the phone circuitry, which is easily overloaded.

LINK TO SCHEMATIC

As seen in the pictures below, the circuit board fits in a compact 3D printed enclosure. The battery sits above the circuit board and is enclosed by a a top cover. I use a rechargeable 9 volt because the device draws about 30mA in total.t I included a 2.5mm circular power plug as a charging jack such that the battery can be charged without removal from a generic 9 volt battery charger.

Picture of home etched Circuit Board:

direct3

Circuit Board mounted in 3D printed Case:

direct2

direct1

 

Link to Demo:

 

My Hell Dice Pedal Board Design

I have been performing way more than I use to! I play club gigs twice a week or more these days…and as a result, my relationship with my rig has really changed. Which is to say, my perspective on what I actually use and what features matter most, has evolved. My conclusions are these: I want small, light weight rugged equipment…I don’t want too many knobs or complexity. I like to use a few basic settings and sounds and that is it.  So I have designed a compact amplifier and set of small 1590a form factor pedals to create a complete amplifier/pedal board rig that weighs a couple of pounds and is about 12 inches long and 4 inches wide. It includes: My 100 watt stomp amp with an auxiliary 9 volt output, a high performance PT2399 type delay, a simple but really nice sounding LDR based envelope filter,  A very pleasing two stage LDR phase shift Vibrato and a hex inverter based overdrive with slightly different approach than the typical design.

My New Compact Pedal Board

pedal board2

As an aside all of these pedals are made with machined(not die cast) Hammond sized aluminum enclosures from Ebay (alpinetech), which I etched and anodized. You cannot properly anodize die cast enclosures because of other ingredients that are mixed with the aluminum…so you have to get CNC milled enclosures if you want to anodize the enclosures.  I discussed my home anodizing process here: Anodizing discussion

More Pictures:funkyfilt

vibrato board

delay board

Links to Schematics and Design Notes:

A few notes: Many of the part choices are  not critical..like transistors, op amps and voltage regulators I use.. The LDR based effects will need tweaking based on the output efficiency of the LED/photo-resistor combo or if you use a commercial opto-coupler instead. The vibrato uses my pic based LFO…but it can easily be replaced with other LFO circuits. The delay uses a 8 pole switch cap filter IC but if desired, this can be replaced by using the unused op amp on the PT2399 IC as a low pass filter.

Compact Stomp Amp:

Hell Dice Delay:

Hell Dice Vibrato:

Hell Dice Overdrive:

Hell Dice Funky Filter:

Pedal Board Demos From Live Show:

Demo of Delay and Vibrato:

Demo of amp clean no effects:

Demo of Envelope Filter(two minutes in):

PWM Vibrato that sounds Awesome!

Vibrato is the slight changing of frequency of a musical note. This can be accomplished by means of a varying time delay in a delay line or by phase shifting and analog signal over time.

The delay approach works fine but creates inherent latency and can sound clownish. So I wanted to create an analog design.

A modulated allpass filter can be used to change phase over time (which is frequency). This analog approach can sound lovely but to sound really good has to linear in its sweep and be modulated  in a sinusoidal manner. Both the design objectives are not trivial.

It occurred to me that my PWM phaser design does all this but simply mixes the original signal back in to create the phasing notches. So I modified the circuit and optimized it to simply modulate the filter and it works great. The PWM driven analog switches are very linear and the PIC DDS works great to create the sine wave modulation. I added some pre- emphasis/de-emphasis, adjusted some values and made an improved layout…so the pedal is super quiet. I left in the peaking filter adjustment to allow for some cool modulated filter effects along with the Vibrato. The range of PWM controls the extent(depth) of the effect at a given modulation rate and so the vibrato pitch bend level is easily adjustable.  Keep in mind the faster the modulation rate; the pitch shift is also greater because a faster the rate of change of phase over a given time, produces more pitch shift.  An alternate analog modulation circuit can be used if desired but the output swing needs to be limited from 0 – 2 volts.

Link to Schematic:

 

Brainwash Phaser converted into Vibrato pedal:

Coming Shortly Demo:

 

Stomp Amp with Improved Tone Stack

Have Just finished designing my new version of the Stomp Amp. I just love it. It has the same 100 watt Class D final as my Red Scare version: https://circuitsalad.com/2017/06/15/100-watt-guitar-amp-pedal/

but with an enhanced tone stack much like that of the Polytone Brute amps and a JFET driver stage for the final. The normal/bright selector is quite useful as well depending on the pickup type and location.

Here is a download link for the schematic and PCB in express PCB format(can be converted to gerber using freeware: copper connection)

https://www.adrive.com/public/ppWwBM/Stomp%20AMP%20Deluxe.zip

 

Final Amp design in 1590BB enclosure:

 

Populated home brew circuit board:

 

Schematic: ( click here for larger view):

Demo coming soon!

100 watt Guitar Amp Pedal

Continuing on my Stomp Amp theme; I have created a 100 watt (24V supply, 4 ohm load) guitar amplifier with FV-1 based DSP reverb and optional treble boost. It fits in a 1590b stomp box. Yes it really is a 100 watt amp!

red_scare

It has Gain, a single tone control, and reverb level control. The reverb room size is set by a resistive divider(R21 , R22) and can also be made adjustable. It utilizes a TPA3116D2 class D amplifier IC which can be configured for mono or stereo output.

Click Here for Large Schematic Image:

Stomp Amp 100_mini

The amplifier sounds delightful. The class D topology provides greater than 90% efficiency. This  eliminates the need for substantial heat sinking. The only penalty is that for guitar applications, pushing the amplifier to distortion does not sound so great. I use an overdrive pedal so I don’t care about this.

Update: I added a LED clipping circuit to make sure the input into the class D final amplifier is level limited(clips/distorts) before the final amplifier starts distorting

Link to Revised Schematic

 

pop_pcbred pcbHome brew laser printer resist circuit board

Completed Circuit Board 

Link To CAD FILES: https://www.adrive.com/public/QpRQMX/RED%20SCARE.zip

The single tone control is surprisingly versatile. It alters the level and center of a MID scoop. You can get really FAT all the way to bright twang all with one control. The circuit  can easily be modified to employ a more sophisticated tone stack if desired. The amplifier requires a 12-24V power supply with at least 4 amp sourcing capability. You can purchase a small lightweight switch-mode supply from Amazon for less than $20.00 that will work nicely. It is ridiculously small, lightweight, loud as hell, and sounds superb through a couple of 10″s or a single 12″  cabinet.

 

Quick Demo of built in Blue LED clipper limiter added to original prototype(you can see the LEDs flash as the input is clipped)