GSM module power supply testing

Most GSM modules that I know need supply voltage in range 3.4 to 4.5[V] with voltage ripples less than 400[mV]. The main issue in designing such a power supply is that it has to maintain correct voltage even during voice call and data transmission. Not going into details current consumption during transmission looks like this:

So you see that surge current pulse takes 577[us] and is followed by about 4 [ms] of "pause". Burst current can reach 2A. I encountered problem with supplying GSM modules couple of times, sometimes clues you get from GSM module which is weakly supplied are really strange, for example Q2687 from Sierra Wireless won't reset when voltage is to low but it won't register to network, either.  Not to make that mistake again I decided to design a tiny little circuit to test voltage regulators designed for GSM modules.

It is as simple as possible - voltage from tested power supply is connected to X1-2 (+) and X1-1 (ground). Q1 MOSFET keys current which depends on supply voltage and value of R1. R1 is big, wire-wound resistor with power rating of 5W. Assuming that circuit is supplied from 4V and not taking into consideration resistance of opened MOSFET surge current should be about 2[A], when Q1 is switched off whole circuit consumes less than 1[mA] to power up PIC microcontroller. D1 diode prevents C1 from draining through R1, thanks to this diode and C1 microcontroller should work even during huge voltage dropouts across X1 terminals. I also added LED1 just to show that circuit is still up and running. You could also use GSM module to generate such distinct power consumption pattern but to do so you have to force module start GPRS session or to make voice call which seems to be impractical, I preffer to have such circuit so I can see both current and voltage on my ocillocope, analize voltage regulator response and do long-term tests under such load.
PCB project looks like this:

and as you can see - it is designed using only one signal side, that way I can make this PCB at home using well knonw method presented here http://www.riccibitti.com/pcb/pcb.htm

Channel 1 below is current measured with a 0.1Ohm resistor and channel 2 is voltage supplied to the real GSM module. Chanel 2 (voltage) had to be inverted because my oscilloscope doesn't have galvanically separated channels. It's easy o calculate that peak power consumption reaches 8W, when we consider low duty cycle average power is less than 2W but duration of peak is rather long - 595us, it's not that easy to store that amount of energy in filtering capacitors .