Reflecting Back

The text blog has been around for about two years now and the video blog is a little over one year old. I thought this would be a great time to look back and tell you a bit more about what happens in the background and what it takes to produce a blog like this.

When I started this blog, the only reader was myself and my wife as proofreader. Starting a blog is completely based on the belief that one day, someone will read and value the stuff you put out there. There’s absolutely no guarantee for this to happen. You may remain the only reader of your own articles for a long while.

Relocating to the US cost me a lot of my valuable lab equipment. One of the most difficult things to get is sponsors when your viewer count is right around 2. So I started by translating some of my old articles into English and began posting them. I was positively surprised to see that just a few month after doing this, views would skyrocket. Of course, the views were ridiculous compared to what they are now.

Small sponsors started submitting demos of their products for me to review. Abracon and freescale were among the first. Analog Devices and STMicroelectronics followed. While I really appreciate those contributions, they were relatively small. I wanted a big sponsor. Not because I am greedy, but because of the fact that I wanted to see someone take a risk on me. If a large manufacturer was willing to pony up a significant amount of cash, I knew that someone else would see the vision. In the beginning of 2013, this sponsor came along, Teledyne LeCroy. And knowing that to date the opinions about my blog within LeCroy are quite divergent, I can only imagine what kind of effort this must have taken by the driving person in this.

The problem with every hobby is that it comes after everything else. Money making first, family second and then the hobby. I know it should be family first, but unless you live in a dream world, you know that family doesn’t work without income. One theoretically easy way to shift the hobby into the “money making” category was to make it profitable. That’s a plan I have started to implement but I am far from it. Google AdSense is cool but even big guys like Dave Jones from the EEVBlog need to create extra revenue to make a living. That’s simply because Google AdSense doesn’t cut it.

Therefore I have implemented a Paypal “donate option” and LeCroy, once again, is helping out, as you can see by the ad in the top right corner. All the revenue together is at least enough to break even and buy better equipment every once in a while. Getting demo devices is no longer a real issue. So this is great progress.

One thing I noticed is that the access to manufacturers of RF components is more difficult than I thought. They often work in tight military dominated markets and are also mostly very ignorant of social media. As I have shown in one of my video, Mini Circuits is the first to hop on board. This is an excellent development. Believe it or not, those little components / modules add up quite quickly.

With the video blog going so well, I have unfortunately ignored the text blog quite a bit. I am not sure if that’s a particularly bad thing or not. The views / subscribers of the text blog are still exponentially rising. But it’s definitely one thing I need to improve. And I will.

So that’s the look into the past of this blog. What’s in the future? Since I don’t have a crystal ball handy, I have absolutely no idea. But I know that the past 1 – 1.5 years have definitely affirmed that my time and money invested in this blog were well spent.

 

Comcast and their Network Management Policy

Not only does Comcast consider it “too much bandwidth” use if their customers use more than 70% of the bandwidth they pay for, for more than 15 minutes, Comcast representatives also have no clue about this policy being in place.

My last YouTube video was rather large. About 4.2 GB to be precise. The upload started out fine and then all of a sudden dropped down to about 1 MBit/s and later to 300 kBit/s. What happened? Network problems? YouTube having problems? I remember the exact same thing happening with older video uploads. So it was something consistent that happens every time I upload a video. After a bit of research, I found that Comcast actually has a Network Management Policy that lines out when and why they throttle bandwidth [1].

Can you give me some “real world” examples of how much bandwidth consumption would be considered too much? For example, how many movies would I have to download to be affected by this congestion management technique?

Since the technique is dynamic and works in real time, the answer really depends on a number of factors including overall usage, time of day and the number of applications a customer might be running at the same time. First, the local network must be approaching a congested state for our technique to even look for traffic to manage. Assuming that is the case, customers’ accounts must exceed a certain percentage of their upstream or downstream (both currently set at 70%) bandwidth for longer than a certain period of time, currently set at 15 minutes.

Alright, so using just 71% of the Bandwidth I pay for, for more than 15 minutes will get my bandwidth throttled to a ridiculously small amount. Naturally, being pissed off about this, I fired a tweet at Comcast:

My initial Tweet directed at Comcast

My initial Tweet directed at Comcast

Now watch this, Comcast is slick. While normal companies respond to problems directly from their main account, Comcast has Mike Lewis. Mike Lewis is a Comcast guy responding to customer complaints. In reality, Mike Lewis is, of course, more than one person. But giving that account a personal touch is their idea of giving the customer a warm and fuzzy feeling, I guess. By using a secondary account, they of course make sure that the main account doesn’t draw any attention to your problem so that other Comcast customers don’t read about your issue.

But the amazing thing is his statement. He insists that they don’t throttle bandwidth. Which is funny, because we just read in their official policy that they do. Interesting, isn’t it?

Mike Lewis from Comcast promptly responds

Mike Lewis from Comcast promptly responds

I gave him some education on the fact. Turns out he didn’t want to help me with this issue anymore after that. Go figure.

And Mike Lewis from Comcast gets a quick lesson on their own policiy

And Mike Lewis from Comcast gets a quick lesson on their own policiy

So what’s the workaround to the problem? Luckily, I have a more advanced router with Quality of Service (QoS) profiles. I pay for 15 MBit/s upload speed. 70% of this is 10.5 MBit/s. So, I set the QoS profile up to never allow the outbound traffic to exceed 10 MBit/s. That way, I can never possibly exceed the 70% condition of Comcast’s Network Management Policy. So even though I throttle myself to about 2/3 of the bandwidth I pay for, this is still a whole lot better than 1 MBit/s or even 300 kBit/s.

It’s still sad, though, that Comcast does heavily limit you this way. If you pay for 15 MBit/s upstream, you expect this to be available 24/7. What’s even more sad is that apparently the customer service representatives at Comcast don’t even have a clue about their own policies.

Links and Sources:
[1] Comcast, Network Management Policy: http://customer.comcast.com/

 

Measuring filter response using a noise source

If you own a spectrum analyzer with tracking generator or, even better, a good vector network analyzer, measuring the frequency response of a filter is fairly easy. But there is one old approach using a noise source that can turn any oscilloscope with a FFT spectrum view into a useful tool for determining the frequency response of a filter.

The concept I will outline in this article is absolutely nothing new. Back in the days, engineers and hobbyists would use analog noise sources in combination with a spectrum analyzer to determine the frequency response of a filter. The concept is very simple: the wide-band noise source is connected to the input of the filter and the output of the filter is connected to the input of the spectrum analyzer. As long as the noise level is somewhat flat over a wide frequency range, one can read the frequency response directly on the spectrum analyzer. So the question is, does this work with a cheap DDS signal generator and a scope with FFT spectrum display, as well?

I used a Rigol DG1022 signal generator as noise source. The generator has a “noise” option. The first thing I wanted to know is how linear the output spectrum of this noise generator is. I captured the spectrum with a Teledyne LeCroy HDO4024. The frequency range shown is 1 MHz to 80 MHz.

Output spectrum of the DG1022's noise function

Output spectrum of the DG1022′s noise function

Looks like it’s pretty usable ’til about 10 MHz. If you need more bandwidth, you either need a better dig gen or you could build a simple analog noise source using a reverse biased Zener diode and a couple of transistor amplifiers. For very high frequencies and high linearity, there are special noise diodes available, as well. But for HF and HF experiments, a Zener diode and a few 2N2222 as amplifiers are more than enough.

The first filter I tested is a bandpass filter for the 40m amateur radio band. Since the filter was designed for a 50 Ohm impedance, care needs to be taken to set the scope and the signal generator to 50 Ohms, as well. The FFT spectrum view of the oscilloscope was set to “Max Hold.” After a few seconds of noise input, this is what I got:

Frequency response of a 40m-Band filter

Frequency response of a 40m-Band filter

But how accurate is the result? Let’s compare.

The following image shows a 10.7 MHz ceramic IF filter measured using the noise setup as described before.

10.7 MHz ceramic IF filter measured using a noise source

10.7 MHz ceramic IF filter measured using a noise source

As you can see, the filter curve is not very smooth, but the general filter parameters are clearly visible. In order to compare the measurements, I used a classic sweep generator setup. The generator swept between 9.7 MHz and 11.7 MHz over a time span of 10 seconds. I picked 10 seconds so that I could simply use the Max Hold function of the oscilloscope’s spectrum analyzer without having to bother about synchronization. The following picture does not only show the results, but it also shows the exact setting I used to set-up the FFT spectrum analysis.

10.7 MHz ceramic IF filter measured using a sweep generator

10.7 MHz ceramic IF filter measured using a sweep generator

The comparison shows that this method words very well to use a modern oscilloscope with FFT spectrum analysis to determine the characteristics of a filter’s behavior in the frequency domain.